Encoding method and decoding method, and device using same

ABSTRACT

The present invention relates to an encoding method and decoding method, and a device using the same. The encoding method according to the present invention comprises the steps of: specifying an intra prediction mode for a current block; and scanning a residual signal by intra prediction of the current block, wherein the step of scanning the residual signal can determine a scanning type for a luminance signal and a chroma signal of the current block according to an intra prediction mode for a luminance sample of the current block.

CROSS-REFERENCE TO RELATED APPLICATIONS

This Application is a Continuation Application of U.S. application Ser.No. 14/388,514, filed on Sep. 26, 2014, which is a national stageapplication under 35 USC 371 of International Application No.PCT/KR2013/003207 filed on Apr. 16, 2013, which claims the benefit ofKorean Application No. 10-2012-0039444 filed on Apr. 16, 2012, KoreanApplication No. 10-2012-0040015 filed on Apr. 17, 2012, KoreanApplication No. 10-2012-0039622 filed on Apr. 17, 2012, KoreanApplication No. 10-2012-0043456 filed on Apr. 25, 2012, and KoreanApplication No. 10-2013-0041906 filed on Apr. 16, 2013, in the KoreanIntellectual Property Office, the entire disclosures of which areincorporated herein by reference for all purposes.

TECHNICAL FIELD

The present invention relates to video encoding and decoding technologyand, more particularly, to a method of determining a frequency transformmethod and/or a scanning method for a chroma signal and an apparatususing the method.

BACKGROUND ART

As broadcast having High Definition (HD) resolution is extended andserved nationwide and worldwide, many users are being accustomed toimages having high resolution and high picture quality. Accordingly,lots of institutes are giving impetus to the development of thenext-image device.

As there is a growing interest in Ultra High Definition (UHD) havingresolution 4 times higher than HDTV along with HDTV, there is a need fortechnology in which an image having higher resolution and higher picturequality is compressed and processed.

In order to compress an image, information about the pixels of a currentpicture can be encoded by prediction. For example, inter-predictiontechnology in which a value of a pixel included in a current picture ispredicted from temporally anterior and/or posterior pictures and/orintra-prediction technology in which a value of a pixel included in acurrent picture is predicted using information about a pixel included inthe current picture can be used.

Pixel information includes a luma signal regarding luma and a chromasignal regarding chroma. A luma signal and a chroma signal may behandled equally or differently depending on the fact that a human being'sense of sight is more sensitive to luma.

Accordingly, a method of effectively handling a chroma signal and a lumasignal separately is problematic.

DISCLOSURE Technical Problem

An object of the present invention is to provide a method and apparatusfor increasing encoding/decoding efficiency in video encoding/decoding.

Another object of the present invention is to provide an efficientfrequency transform method and apparatus for a chroma signal and anefficient scanning method and apparatus for a chroma signal.

Technical Solution

In an aspect of the present invention, in selecting a frequencytransform method and a scanning method for a chroma signal, a differentfrequency transform method can be used depending on an intra-predictionmode of a chroma signal and a different scanning method may bedetermined depending on an intra-prediction mode.

In another aspect of the present invention, a scanning type for the lumasignal and the chroma signal of a current block can be determined by anintra-prediction mode for the luma sample of the current block.

Here, a scanning type for a luma signal can be equally used as ascanning type for a chroma signal. In another embodiment, a scanningtype for a chroma signal may be determined using the same method as amethod of determining a scanning type for a luma signal can be used.

Advantageous Effects

In accordance with the present invention, encoding/decoding efficiencycan be improved in video encoding/decoding. More particularly, accordingto the present invention, frequency transform and/or scan for a chromasignal can be performed efficiently.

Furthermore, in selecting a frequency transform method and a scanningmethod for a chroma signal, the encoding efficiency of the residualsignal of the chroma signal can be improved using a method and apparatusfor applying a frequency transform method to the chroma signal dependingon an intra-prediction direction mode of the chroma signal and derivinga scanning direction for the chroma signal.

DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram showing the construction of a video encodingapparatus in accordance with an embodiment of the present invention.

FIG. 2 is a block diagram showing the construction of a video decodingapparatus in accordance with an embodiment of the present invention.

FIG. 3 shows a brief example of a method of partitioning a CU within anLCU when encoding a picture.

FIG. 4 schematically illustrating examples regarding the partitionstructures of a PU.

FIG. 5 shows a brief example of the partition structure of TUs within aCU.

FIG. 6 schematically illustrates intra-prediction modes.

FIG. 7 is a diagram schematically illustrating the relationship ofEquation 1.

FIG. 8 is a diagram illustrating an example of up-right diagonalscanning for important group flags and transform coefficients.

FIG. 9 is a flowchart illustrating an example of a method of determininga scanning direction according to a frame prediction direction.

FIG. 10 is a flowchart schematically illustrating another example of amethod of determining a scanning direction according to a frameprediction direction.

FIG. 11 is a flowchart illustrating an example of a method of selectinga frequency transform method for a residual picture.

FIG. 12 is a flowchart illustrating an example of a method of deriving ascanning direction for a chroma signal according to the presentembodiment.

FIG. 13 is a flowchart illustrating another example of a method ofderiving a scanning direction for a chroma signal according to thepresent embodiment.

FIG. 14 is a flowchart illustrating an embodiment of a method ofderiving a scanning direction for a chroma signal according to thepresent embodiment.

FIG. 15 is a flowchart illustrating another embodiment of a method ofderiving a scanning direction for a chroma signal according to thepresent embodiment.

FIG. 16 is a flowchart illustrating an example of a method ofselectively deriving a frequency transform method for the residualpicture of a chroma signal based on an intra-prediction direction modefor the chroma signal according to the present embodiment.

FIG. 17 is a flowchart illustrating another example of a method ofselectively deriving a frequency transform method for the residualpicture of a chroma signal based on an intra-prediction direction modefor the chroma signal according to the present embodiment.

FIG. 18 is a flowchart illustrating an example of a method ofselectively deriving a frequency transform method for the residualpicture of a chroma signal based on an intra-prediction direction modefor a luma signal in relation to an Intra_FromLuma (LM) encoding modefor the chroma signal according to the present embodiment.

FIG. 19 is a flowchart illustrating another example of a method ofselectively deriving a frequency transform method for the residualpicture of a chroma signal based on an intra-prediction direction modefor a luma signal in relation to the Intra_FromLuma (LM) encoding modeof the chroma signal according to the present embodiment.

FIG. 20 is a flowchart illustrating an example of a method ofselectively deriving a frequency transform method for the residualpicture of a chroma signal based on an intra-prediction direction modeaccording to the present embodiment.

FIG. 21 is a flowchart illustrating another example of a method ofselectively deriving a frequency transform method for the residualpicture of a chroma signal based on an intra-prediction direction modeaccording to the present embodiment.

FIG. 22 shows an example of a difference in the resolution between aluma block and a chroma block.

FIG. 23 is another example illustrating a difference in the resolutionbetween a luma block and a chroma block.

FIG. 24 shows another example of an encoding apparatus according to thepresent invention.

FIG. 25 shows another example of a decoding apparatus according to thepresent invention.

MODE FOR INVENTION

Some exemplary embodiments of the present invention are described indetail with reference to the accompanying drawings. Furthermore, indescribing the embodiments of this specification, a detailed descriptionof the known functions and constitutions will be omitted if it is deemedto make the gist of the present invention unnecessarily vague.

In this specification, when it is said that one element is ‘connected’or ‘coupled’ with the other element, it may mean that the one elementmay be directly connected or coupled with the other element or a thirdelement may be ‘connected’ or ‘coupled’ between the two elements.Furthermore, in this specification, when it is said that a specificelement is ‘included’, it may mean that elements other than the specificelement are not excluded and that additional elements may be included inthe embodiments of the present invention or the scope of the technicalspirit of the present invention.

Terms, such as the first and the second, may be used to describe variouselements, but the elements are not restricted by the terms. The termsare used to only distinguish one element from the other element. Forexample, a first element may be named a second element without departingfrom the scope of the present invention. Likewise, a second element maybe named a first element.

Furthermore, element modules described in the embodiments of the presentinvention are independently shown to indicate different andcharacteristic functions, and it does not mean that each of the elementmodules is formed of a piece of separate hardware or a piece ofsoftware. That is, the element modules are arranged and included, forconvenience of description, and at least two of the element modules mayform one element module or one element may be divided into a pluralityof element modules and the plurality of divided element modules mayperform functions. An embodiment into which the elements are integratedor embodiments from which some elements are separated are also includedin the scope of the present invention, unless they depart from theessence of the present invention.

Furthermore, in the present invention, some elements are not essentialelements for performing essential functions, but may be optionalelements for improving only performance. The present invention may beimplemented using only essential elements for implementing the essenceof the present invention other than elements used to improve onlyperformance, and a structure including only essential elements otherthan optional elements used to improve only performance is included inthe scope of the present invention.

FIG. 1 is a block diagram showing the construction of a video encodingapparatus in accordance with an embodiment of the present invention.

Referring to FIG. 1, the video encoding apparatus 100 includes a motionprediction module 111, a motion compensation module 112, anintra-prediction module 120, a switch 115, a subtractor 125, a transformmodule 130, a quantization module 140, an entropy encoding module 150, adequantization module 160, an inverse transform module 170, an adder175, a filter module 180, and a reference picture buffer 190.

The video encoding apparatus 100 can perform encoding on an inputpicture in intra-mode or inter-mode and output a bit stream as a resultof the encoding. In this specification intra-prediction has the samemeaning as intra-frame prediction, and inter-prediction has the samemeaning as inter-frame prediction. In the case of intra-mode, the switch115 can switch to intra mode. In the case of inter-mode, the switch 115can switch to inter-mode. The video encoding apparatus 100 can generatea prediction block for the input block of an input picture and thenencode the residual between the input block and the prediction block.Here, the input picture can mean the original picture.

In the case of intra-mode, the intra-prediction module 120 can generatethe prediction block by performing spatial prediction using values ofthe pixels of an already encoded block neighboring a current block.

In the case of inter-mode, the motion prediction module 111 can obtain amotion vector by searching a reference picture, stored in the referencepicture buffer 190, for a region that is most well matched with theinput block in a motion prediction process. The motion compensationmodule 112 can generate the prediction block by performing motioncompensation using the motion vector and the reference picture stored inthe reference picture buffer 190. Here, the motion vector is atwo-dimensional (2-D) vector used in inter-prediction, and the motionvector can indicate an offset between a current block and a block withina reference picture.

The subtractor 125 can generate a residual block based on the residualbetween the input block and the generated prediction block. Thetransform module 130 can perform transform on the residual block andoutput a transform coefficient according to the transformed block.Furthermore, the quantization module 140 can output a quantizedcoefficient by quantizing the received transform coefficient using atleast one of a quantization parameter and a quantization matrix. Here,the quantization matrix can be inputted to an encoder, and the inputtedquantization matrix can be determined to be used in the encoder.

The entropy encoding module 150 can perform entropy encoding based onvalues calculated by the quantization module 140 or an encodingparameter value calculated in an encoding process and output a bitstream according to the entropy encoding. If entropy encoding is used,the size of a bit stream for a symbol to be encoded can be reducedbecause the symbol is represented by allocating a small number of bitsto a symbol having a high incidence and a large number of bits to asymbol having a low incidence. Accordingly, the compression performanceof video encoding can be improved through entropy encoding. The entropyencoding module 150 can use such encoding methods as exponential Golomb,Context-Adaptive Binary Arithmetic Coding (CABAC), and Context-AdaptiveBinary Arithmetic Coding (CABAC) for the entropy encoding.

The video encoding apparatus according to the embodiment of FIG. 1performs inter-prediction encoding, that is, inter-frame predictionencoding, and thus a currently encoded picture needs to be decoded andstored in order to be used as a reference picture. Accordingly, aquantized coefficient is dequantized by the dequantization module 160and is then inversely transformed by the inverse transform module 170.The dequantized and inversely transformed coefficient is added to theprediction block through the adder 175, thereby generating areconstructed block.

The reconstructed block experiences the filter module 180. The filtermodule 180 can apply one or more of a deblocking filter, a SampleAdaptive Offset (SAO), and an Adaptive Loop Filter (ALF) to thereconstructed block or the reconstructed picture. The filter module 180may also be called an adaptive in-loop filter. The deblocking filter canremove block distortion generated at the boundary of blocks. The SAO canadd a proper offset value to a pixel value in order to compensate for acoding error. The ALF can perform filtering based on a value obtained bycomparing a reconstructed picture with the original picture. Thereconstructed block that has experienced the filter module 180 can bestored in the reference picture buffer 190.

FIG. 2 is a block diagram showing the construction of a video decodingapparatus in accordance with an embodiment of the present invention.

Referring to FIG. 2, the video decoding apparatus 200 includes anentropy decoding module 210, a dequantization module 220, an inversetransform module 230, an intra-prediction module 240, a motioncompensation module 250, a filter module 260, and a reference picturebuffer 270.

The video decoding apparatus 200 can receive a bit stream outputted froman encoder, perform decoding on the bit stream in intra-mode orinter-mode, and output a reconstructed picture, that is, a restoredpicture. In the case of intra-mode, a switch can switch to intra-mode.In the case of inter-mode, the switch can switch to inter-mode. Thevideo decoding apparatus 200 can obtain a reconstructed residual blockfrom the received bit stream, generate a prediction block, and thengenerate a reconstructed block, that is, a restored, by adding thereconstructed residual block to the prediction block.

The entropy decoding module 210 can generate symbols including a symbolhaving a quantized coefficient form by performing entropy decoding onthe received bit stream according to a probability distribution. In thiscase, an entropy decoding method is similar to the aforementionedentropy encoding method.

If an entropy decoding method is used, the size of a bit stream for eachsymbol can be reduced because the symbol is represented by allocating asmall number of bits to a symbol having a high incidence and a largenumber of bits to a symbol having a low incidence.

The quantized coefficient is dequantized by the dequantization module220 and is inversely transformed by the inverse transform module 230. Asa result of the dequantization/inverse transform of the quantizedcoefficient, a reconstructed residual block can be generated.

The quantization matrix used in dequantization is also called a scalinglist. The dequantization module 220 can generate a dequantizedcoefficient by applying the quantization matrix to a quantizedcoefficient.

Here, the dequantization module 220 can perform dequantization inresponse to quantization used in an encoder. For example, thedequantization module 220 can perform dequantization by inverselyapplying a quantization matrix, used in an encoder, to a quantizedcoefficient.

The quantization matrix used in dequantization in the video decodingapparatus 200 may be received from the bit stream, or a default matrixalready included in an encoder and/or a decoder may be used as thequantization matrix used in dequantization in the video decodingapparatus 200. Information about a transmitted quantization matrix canbe received through a sequence parameter set or a picture parameter setfor each transform block size to which a quantization matrix size or aquantization matrix is applied. For example, 4×4 quantization matricesfor a 4×4 transform block can be reduced, 8×8 matrices for an 8×8transform block can be reduced, 16×16 matrices for a 16×16 transformblock can be reduced, and 32×32 matrices for a 32×32 transform block canbe reduced.

In the case of intra-mode, the intra-prediction module 240 can generatea prediction block by performing spatical prediction using pixel valuesof an already decoded block neighboring a current block. In the case ofinter-mode, the motion compensation module 250 can generate a predictionblock by performing motion compensation using a motion vector and areference picture stored in the reference picture buffer 270.

The residual block and the prediction block are added together by anadder 255. The added block experiences the filter module 260. The filtermodule 260 can apply at least one of a deblocking filter, an SAO, and anALF to the reconstructed block or the reconstructed picture. The filtermodule 260 outputs a reconstructed picture, that is, a reconstructedpicture. The reconstructed picture can be stored in the referencepicture buffer 270 and can be used for inter-frame prediction.

In HEVC, in order to encode a picture efficiently, encoding can beperformed in a Coding Unit (hereinafter referred to as a ‘CU’).

FIG. 3 shows a brief example of a method of partitioning a CU within aLargest Coding Unit (hereinafter referred to as an ‘LCU’) when encodinga picture.

A picture inputted to the encoding apparatus is sequentially partitionedper LCU as in FIG. 3, and a partition structure can be determined in theLCU.

The partition structure means a distribution of CUs for efficientlyencoding a picture within an LCU. The distribution of the CUs can bespecified by determining whether or not each CU will be partitioned intofour CU the size of which has been reduced by half horizontally andvertically. Likewise, the partitioned CU can be recursively partitionedinto four CUs the size of which has been reduced by half horizontallyand vertically.

Here, the partition of the CU may be performed up to a predetermineddepth. Information about the depth indicates the size of a CU, and theinformation is stored in all the CUs. The depth of an LCU that is abasis is 0, and the depth of a Smallest Coding Unit (hereinafterreferred to as an ‘SCU’) is a predetermined maximum depth.

The depth of a CU is increased by 1 whenever partition is performed byhalf from an LCU horizontally and vertically. A CU on which partitionhas not been performed in each depth has a 2N×2N size. If partition isperformed on a CU, the CU having a 2N×2N size before the partition ispartitioned into four CUs each having an N×N size.

The size of a CU is reduced to half whenever the depth of the CU isincreased by 1. FIG. 3 shows an example in which the size of an LCUhaving a minimum depth of 0 is a 64×64 pixel and the size of an SCUhaving a maximum depth of 3 is an 8×8 pixel.

In the example of FIG. 3, the CU (LCU) of the 64×64 pixel has a depth of0, the CU of a 32×32 pixel has a depth of 1, the CU of a 16×16 pixel hasa depth of 2, and the CU (SCU) of the 8×8 pixel has a depth of 3.

Furthermore, information about whether or not a CU will be partitionedis represented by 1-bit partition information for each CU. The partitioninformation is included in all CUs other than an SCU. If a CU is notpartitioned, a value of the partition information can be set to 0. If aCU is partitioned, a value of the partition information can be set to 1.

A Prediction Unit (hereinafter referred to as a ‘PU’) is a unit ofprediction.

FIG. 4 schematically illustrating examples regarding the partitionstructures of a PU.

As in the example of FIG. 4, one CU can be partitioned into several PUs,and prediction can be performed on the PUs.

A Transform Unit (hereinafter referred to as a ‘TU’) is a basic unitused in a spatial transform and quantization process within a CU. A TUcan have a rectangular or square form.

Each CU can have one or more TU blocks, and the TU block has a quad-treestructure.

FIG. 5 shows a brief example of the partition structure of TUs within aCU. As shown, TUs within a CU 510 can have various sizes depending on aquad-tree structure.

In intra-prediction encoding/decoding, prediction encoding according todirectivity can be performed from blocks neighboring a current block.

In intra-prediction, encoding/decoding are performed using a total of 36prediction modes including 33 angular prediction modes and 3 non-angularprediction modes.

FIG. 6 schematically illustrates intra-prediction modes.

From among the 36 intra-prediction direction modes, the 3 modes withoutdirectivity include a planar mode Intra_Planar, a mean mode ‘Intra_DC(DC)’, and a mode ‘Intra_FromLuma (LM)’ in which a chroma signal ispredicted from a reconstructed luma signal. In intra-prediction, all the3 non-angular modes may be used or only some of the 3 non-angular modesmay be used. For example, only the planar mode and the mean mode may beused and the LM may not be used.

Encoding for the 36 intra-prediction direction modes can be applied toeach of a luma signal and a chroma signal. In the case of a luma signal,the LM can be excluded. In the case of a chroma signal, encoding on anintra-prediction direction mode can be performed according to threemethods as in Table 1.

TABLE 1 Intra-prediction direction mode for Intra-prediction directionmode for luma signal chroma signal 0 26 10 1 X(0 <= x < 35) 0 34 0 0 0 0(PLANAR) EM 1 26 34 26 26 26 (VER) 2 10 10 34 10 10 (HOR) 3 1 1 1 34 1(DC) 4 LM LM LM LM LM LM 5 0 26 10 1 X DM

Table 1 includes a Derived Mode (DM) in which an intra-predictiondirection mode for a luma signal is used as an intra-predictiondirection mode for a chroma signal without change and an Explicit Mode(EM) in which a real intra-prediction direction mode is encoded.Intra-prediction direction modes for a chroma signal encoded in the EMinclude a planar mode PLANAR, a mean mode DC, a horizontal mode HOR, avertical mode VER, and a mode located at an eighth place in a verticaldirection, that is, Ver+8 or No. 34 mode. There is the LM in which achroma signal is predicted from a reconstructed luma signal.

A mode having the best efficiency can be selected from the three modeencoding/decoding methods; DM, EM, and LM.

A method ‘Intra_FromLuma’ of predicting a chroma signal from areconstructed luma signal sample may be used as a method of performingencoding/decoding by predicting a chroma signal. In this technique, alinear correlation between a chroma signal and a luma signal is used.Here, an example of the used linear correlation is illustrated inEquation 1.

Pred_(c) [x,y]=α·Rec_(L) ‘[x,y]+β  <Equation 1>

FIG. 7 is a diagram schematically illustrating the relationship ofEquation 1.

Pred_(c)[x,y] of Equation 1 indicates a predicted value of a chromasignal in FIG. 7, and Rec_(L)[x,y] indicates a value calculated from aluma signal according to Equation 2 in order to satisfy a 4:2:0 samplingratio of a chroma signal.

Rec_(L) ‘[x,y]=(Rec_(L)[2x,2y]+Rec_(L)[2x,2y+1])>>1   <Equation 2>

In Equation 2, a α value and a β value indicate weight α and acompensation value β between a down-sampled luma signal and adown-sampled chroma signal.

A residual picture, that is, a difference with the original picture, canbe generated using a prediction picture for the chroma signal obtainedaccording to Equations 1 and 2.

The residual picture is subject to frequency domain transform,quantization, and entropy encoding. Furthermore, the frequency domaintransform can include integer transform, integer Discrete CosineTransform (DCT), integer Discrete Sine Transform (DST), orintra-prediction mode-dependent DCT/DST.

As described above, the residual pictures of the original picture andthe prediction picture can be subject to frequency domain transform andquantization and the subject to entropy encoding. Here, in order toincrease entropy encoding efficiency, coefficients having a 2-D formquantized picture can be rearranged into coefficients having a 1-D form.

In a prior art, a zigzag scanning method is used. In contrast, in thisspecification, an up-right diagonal scanning method other than thezigzag scanning method can be used as a scanning method for quantizationcoefficients. Furthermore, frequency domain transform can includeinteger transform, integer DCT, integer DST, or intra-predictionmode-dependent DCT/DST.

Quantization coefficients for a specific block can be grouped into 4×4subblocks and then encoded/decoded.

FIG. 8 is a diagram illustrating an example of an up-right diagonalscanning method for important group flags and transform coefficients.

FIG. 8 shows an example in which a 16×16 block is partitioned into 164×4 subblocks and the 16 4×4 subblocks are encoded.

FIG. 8(a) shows a basic scanning method, and FIG. 8(b) schematicallyshows a case where quantization coefficients are actually scanned andthe results of the scanning.

In a decoding process, whether or not a transform coefficient is presentin each subblock can be checked through significant_coeff_group_flag(sigGrpFlag) parsed from a bit stream. When a value ofsignificant_coeff_group_flag is 1, it means that any one quantizedtransform coefficient is present in a corresponding 4×4 subblock. Incontrast, when a value of significant_coeff_group_flag is 0, it meansthat a quantized transform coefficient is not present in a corresponding4×4 subblock. Up-right scanning is basically applied to a scanningdirection for the 4×4 subblock in FIG. 8 and a scanning direction forsignificant_coeff_group_flag.

FIG. 8 shows an example in which an up-right diagonal scanning method isapplied, but a scanning method for quantization coefficients includes anup-right diagonal scanning method, a horizontal scanning method, and avertical scanning method.

In inter-prediction, the up-right diagonal scanning method can bebasically used. In intra-prediction, the up-right diagonal scanningmethod, the horizontal scanning method, and the vertical scanning methodcan be selectively used.

In intra-prediction, a different scanning direction can be selecteddepending on an intra-prediction direction and may be applied to both aluma signal and a chroma signal.

Table 2 illustrates an example of a method of determining a scanningdirection depending on a frame prediction direction.

TABLE 2

 2-5 0 0 0 0  6-14 2 2 0 0 15-21 0 0 0 0 22-30 1 1 0 0 31-35 0 0 0 0

indicates data missing or illegible when filed

In Table 2, ‘IntraPredModeValue’ indicates an intra-predictiondirection. The intra-prediction direction corresponds to anIntraPredMode value in a luma signal and corresponds to anIntraPredModeC value in a chroma signal. Furthermore, ‘log2TrafoSize’indicates that the size of a current transform block is representedusing ‘log’.

For example, when IntraPredModeValue is 1, it means an ‘Intra_DC’ mode.When ‘log2TrafoSize-2’ is 1, it means an 8×8 block.

Furthermore, in Table 2, numbers 0, 1, or 2 determined byIntraPredModeValue and log2TrafoSize specifies a scanning direction. Forexample, in Table 2, 2 indicates up-right diagonal scanning, 1 indicateshorizontal scanning, and 2 indicates vertical scanning.

FIG. 9 is a flowchart illustrating an example of a method of determininga scanning direction according to a frame prediction direction.

In FIG. 9, IntraPredMode indicates an intra-prediction direction modefor a luma signal, and IntraPredModeC indicates an intra-predictiondirection mode for a chroma signal. IntraPredMode(C) may be a lumasignal or a chroma signal depending on the component of the signal.ScanType indicates a residual signal scanning direction and includesup-right (DIAG; Up-right=0) scanning, horizontal (HOR; Horizontal=1)scanning, and vertical (VER; Vertical=2) scanning.

The example of FIG. 9 can be performed in an encoder apparatus and adecoder apparatus. Furthermore, the example of FIG. 9 may be performedwithin a specific module within the encoder apparatus and the decoderapparatus.

FIG. 10 is a flowchart schematically illustrating another example of amethod of determining a scanning direction according to a frameprediction direction.

In the example of FIG. 10, a scanning method for a luma signal and ascanning method for a chroma signal are differently represented. In FIG.10, an index is an indicator indicating the size of transform, and anindex value according to each transform size can be calculated asfollows.

If a transform size is 64×64, an index is 1. If a transform size is32×32, an index is 2. If a transform size is 16×16, an index is 3. If atransform size is 8×8, an index is 4. If a transform size is 4×4, anindex is 5. If a transform size is 2×2, an index is 6.

From FIG. 10, it can be seen that scanning according to anintra-prediction direction is applied to an 8×8 transform size and a 4×4transform size in the case of a luma signal and is applied to a 4×4transform size, that is, the smallest size, in the case of a chromasignal. Furthermore, if scanning according to an intra-predictiondirection is applied, up-right (DIAG; Up-right=0) scanning can beapplied.

In order to encode quantization coefficients in intra-prediction, ascanning direction obtained from Table 2 can be used as a scanningdirection for a 4×4 subblock and a scanning direction forsignificant_coeff_group_flag as in FIG. 8.

As described above, in an encoding process, a residual picture for theoriginal picture and the prediction picture are subject to frequencydomain transform and quantization and then subject to entropy encoding.Here, in order to increase encoding efficiency due to the frequencydomain transform, integer transform, integer DCT, integer DST, orintra-prediction mode-dependent DCT/DST can be applied selectively andadaptively depending on the size of a block.

Furthermore, in a decoding process, a residual picture is subject toentropy decoding, dequantization, and frequency domain inversetransform. Here, integer inverse transform, integer inverse DCT, integerinverse DST, or intra-prediction mode-dependent DCT/DST can be appliedselectively and adaptively depending on the size of a block.

FIG. 11 is a flowchart illustrating an example of a method of selectinga frequency transform method for a residual picture.

First, if a current block has been subjected to intra-frame encoding andis not a block of a luma signal, integer transform or integer DCT isperformed in a frequency transform method for the residual pictures ofthe luma signal and the chroma signal of the current block.

If not (i.e., the current block has been subjected to intra-frameencoding and is a block of a luma signal), an intra-prediction directionmode ‘IntraPredMode’ for the luma signal of the current block isobtained.

Next, it is checked whether or not the current block is a block having a4×4 size (iWidth==4). If, as a result of the check, the current block isnot a block having a 4×4 size (iWidth==4), integer transform or integerDCT is performed in a frequency transform method for the residualpictures of the luma signal and the chroma signal of the current block.

If, as a result of the check, the current block is a block having a 4×4size (iWidth==4), an intra-prediction direction mode of the currentblock is checked.

If, as a result of the check, the intra-prediction direction mode of thecurrent block is 2 or more and 10 or less, integer DST is performed in afrequency transform method for the luma signal of the current block in ahorizontal direction and integer DCT is performed in a frequencytransform method for the luma signal of the current block in a verticaldirection.

Integer DCT is performed in a frequency transform method for the chromasignal of the current block in both horizontal and vertical directions.

If, as a result of the check, the intra-prediction direction mode of thecurrent block is 0 or is 11 or more and 25 or less, integer DST isperformed in a frequency transform method for the luma signal of thecurrent block in both horizontal and vertical directions.

Integer DCT is performed in a frequency transform method for the chromasignal of the current block in both horizontal and vertical directions.

If, as a result of the check, the intra-prediction direction mode of thecurrent block is 26 or more and 34 or less, integer DCT is performed ina horizontal direction and integer DST is performed in a verticaldirection in a frequency transform method for the luma signal of thecurrent block.

Integer DCT is performed in both horizontal and vertical directions in afrequency transform method for the chroma signal of the current block.

If not, integer DCT is performed in both horizontal and verticaldirections in a frequency transform method for the residual pictures ofthe luma signal and the chroma signal of the current block.

In FIG. 11, iWidth is an indicator indicating the size of transform, anda value of iWidth according to each transform size can be calculated asfollows.

If a transform size is 64×64, iWidth=64. If a transform size is 32×32,iWidth=32. If a transform size is 16×16, iWidth=16. If a transform sizeis 8×8, iWidth=8. If a transform size is 4×4, iWidth=4. If a transformsize is 2×2, iWidth=2.

In relation to FIG. 11, a transform process for scaled transformcoefficients is as follows.

<Transform Process for Scaled Transform Coefficients>

-   -   In this case, the input is as follows.    -   The width of a current transform block: nW    -   The height of the current transform block: nH    -   An array of scaled transform coefficients having an element dij:        (nWxnH) array d    -   An index for the luma signal and the chroma signal of a current        block: cIdx    -   If cIdx is 0, it means a luma signal, and if cIdx is 1 or cIdx        is 2, it means a chroma signal. Furthermore, if cIdx is 1, it        means Cb in a chroma signal, and if cIdx is 2, it means Cr in a        chroma signal.

In this case, the output is as follows.

-   -   An array for a residual signal obtained by inversely        transforming the scaled transform coefficients: (nWxnH) array r

If an encoding mode ‘PredMode’ for the current block is anintra-prediction mode ‘Intra’, a value of Log2(nW*nH) is 4, and a valueof cIdx is 0, parameters horizTrType and vertTrType according to Table 3are used depending on intra-prediction direction modes for a lumasignal. If not, the parameters horizTrType and vertTrType are set to 0.

TABLE 3 IntraPredMode 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17vertTrType 1 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 horizTrType 1 0 1 1 1 1 11 1 1 1 1 1 1 1 1 1 1 IntraPredMode 18 19 20 21 22 23 24 25 26 27 28 2930 31 32 33 34 vertTrType 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 horizTrType1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0

An inverse transform process is performed on the scaled transformcoefficients using the parameters horizTrType and vertTrType. First, thesizes nW and nH and scaled transform coefficients array ‘nWxnH array d’of the current block and the parameter horizTrType are received, 1-Dinverse transform is performed in a horizontal direction using them, andan array ‘nWxnH array e’ is outputted.

Next, the array ‘nWxnH array e’ is received, and an array ‘nWxnH arrayg’ is derived as in Equation 3.

g _(ij)=Clip3(−32768,32767,(e _(ij)+64)>>7)   <Equation 3>

Next, the sizes nW and nH and array ‘nWxnH array g’ of the current blockand the parameter vertTrType are received, and 1-D inverse transform isperformed in a vertical direction using them.

Next, an array ‘nWxnH array r’ for the residual signals is set based oncIdx as in Equation 4.

r _(ij)=(f _(ij)+(1<<(shift-1)))>>shift   <Equation 4>

In Equation 4, ‘shift’=20—BitDepthy when cIdx is 0. ‘shift’=20—BitDepthcwhen cIdx is not 0. BitDepth indicates the number of bits (e.g., 8 bits)for a current picture.

As described above, a method (Intra_FromLuma) of predicting a chromasignal from a reconstructed luma signal sample may be used as a methodof predicting a chroma signal.

A residual signal (an error signal: residual) for a chroma componentencoded using the Intra_FromLuma method is basically scanned in anup-right (Up-right=0) direction and then encoded because there is nointra-frame direction mode.

However, since the Intra_FromLuma method is based on similarity betweena luma signal and a chroma signal, there is a good possibility that aresidual signal for the chroma signal obtained by the Intra_FromLumamethod may have a similar characteristic to a residual signal for theluma signal. Accordingly, encoding efficiency for the residual signal ofthe chroma signal can be improved using this characteristic.

Furthermore, encoding efficiency can be improved by differently applyinginteger DCT and integer DST to a 4×4 luma signal block in horizontal andvertical directions depending on an intra-prediction direction mode of aluma signal. In contrast, only integer DCT is performed on aconventional 4×4 chroma signal block.

There is a possibility that the residual picture of a chroma signal mayhave a similar characteristic to the residual picture of a luma signal.Accordingly, in order to improve encoding efficiency, a selectivefrequency transform method based on an intra-prediction direction modeapplied to the residual picture of the luma signal can also be appliedto the residual picture of the chroma signal.

Furthermore, the method (Intra_FromLuma) of predicting a chroma signalfrom a reconstructed luma signal sample, that is, a method of predict achroma signal, is based on similarity between the luma signal and thechroma signal. Accordingly, there is a good possibility that a residualsignal for the chroma signal obtained by the Intra_FromLuma method has asimilar characteristic to a residual signal for the luma signal. As aresult, if a selective frequency transform method is applied to theresidual signal of the chroma signal based on the above characteristic,encoding efficiency can be improved.

As described above, the Intra_FromLuma method is an intra-predictionmethod using similarity between a luma signal and a chroma signal. Ascanning direction for the residual signal of a chroma signal can bederived from an intra-prediction direction for a luma signal based onthe similarity between the luma signal and the chroma signal.

In this specification, a method and apparatus for deriving a scanningdirection for a chroma signal are described in more detail below.

[Embodiment 1] A method and apparatus for deriving a scanning directionfor a chroma signal based on an intra-prediction direction mode of aluma signal

FIG. 12 is a flowchart illustrating an example of a method of deriving ascanning direction for a chroma signal according to the presentembodiment.

In the example of FIG. 12, the derivation of a scanning direction for achroma signal can be determined by an intra-prediction direction mode ofa luma signal. In other words, a scanning direction for a luma signalcan be applied to a chroma signal without change. Accordingly, there isan advantage in that complexity is reduced because it is not necessaryto derive calculation for deriving a scanning direction for a chromasignal.

The operation of FIG. 12 can be performed in the encoding apparatus ofFIG. 1 and the decoding apparatus of FIG. 2.

Referring to FIG. 12, whether or not the luma component of a currentblock has been subjected to intra-prediction is determined at stepS1210. If, as a result of the determination, it is determined that thecurrent block has not been subjected to intra-prediction (i.e.,subjected to inter-prediction), up-right diagonal scanning is performedon the chroma component and the luma component of the current block atstep S1260. That is, up-right diagonal scanning is performed on both ascan type for the luma component of the current block and a scan typefor the chroma component of the current block.

If, as a result of the determination, it is determined that the lumacomponent of the current block has been subjected to intra-prediction,whether or not an intra-prediction mode for the luma component of thecurrent block is 6 or more and 14 or less is determined at step S1220.That is, whether an intra-prediction mode for the luma component of thecurrent block is an intra-prediction mode in a horizontal direction oran intra-prediction mode in a direction close to horizontality isdetermined at step S1220.

If, as a result of the determination, it is determined that theintra-prediction mode for the luma component of the current block is 6or more and 14 or less, vertical scanning is performed on the lumacomponent and the chroma component of the current block at step S1240.That is, vertical scanning is performed on both a scan type for the lumacomponent of the current block and a scan type for the chroma componentof the current block.

If the luma component of the current block has been subjected tointra-prediction, but it is determined that the intra-prediction modefor the luma component of the current block is not 6 or more and 14 orless, whether or not an intra-prediction mode for the luma component ofthe current block is 22 or more and 30 or less is determined at stepS1230. That is, whether the intra-prediction mode for the luma componentof the current block is an intra-prediction mode in a vertical directionor an intra-prediction mode in a direction close to verticality isdetermined.

If, as a result of the determination, it is determined that theintra-prediction mode for the luma component of the current block is 22or more and 30 or less, horizontal scanning is performed on the lumacomponent and the chroma component of the current block at step S1250.That is, horizontal scanning is performed on both a scan type for theluma component of the current block and a scan type for the chromacomponent of the current block.

If the luma component of the current block has been subjected tointra-prediction, but it is determined that the intra-prediction modefor the luma component of the current block is not 6 or more and 14 orless and is not 22 or more and 30 or less, up-right diagonal scanning isperformed on the luma component and the chroma component of the currentblock at step S1260.

FIG. 13 is a flowchart illustrating another example of a method ofderiving a scanning direction for a chroma signal according to thepresent embodiment. The example of FIG. 13 can have an advantage in thata scanning method for a luma signal and a scanning method for a chromasignal are unified.

The operation of FIG. 13 can be performed in the encoding apparatus ofFIG. 1 and the decoding apparatus of FIG. 2.

Referring to FIG. 13, first, whether or not a current block has beensubjected to intra-prediction is determined at step S1310.

If, as a result of the determination, it is determined that a currentblock has not been subjected to intra-prediction, up-right diagonalscanning is performed on the luma signal and the chroma signal of thecurrent block at step S1390.

If, as a result of the determination, it is determined that the currentblock has been subjected to intra-prediction, the size of the currentblock is changed into an index at step S1320. For example, an index tospecify the size of the current block can be set.

Next, information about the intra-prediction of the current block mode,that is, IntraPredMode, can be obtained at step S1330. IntraPredMode,that is, information indicative of an intra-prediction mode for thecurrent block, can be obtained based on FIG. 6.

Whether or not the index set for the current block at step S1320 isgreater than 3 and smaller than 6 is determined at step S1340. If, as aresult of the determination, it is determined that the index for thecurrent block is not greater than 3 and smaller than 6, up-rightdiagonal scanning is performed on the luma signal and the chroma signalof the current block at step S1390.

If, as a result of the determination, it is determined that the indexfor the current block is greater than 3 and smaller than 6, whether ornot an intra-prediction mode for the chroma signal of the current blockis 6 or more and 14 or less is determined at step S1350. If, as a resultof the determination, it is determined that the intra-prediction modefor the chroma signal of the current block is 6 or more and 14 or less,vertical scanning is performed on the chroma signal of the current blockat step S1370.

If, as a result of the determination, it is determined that theintra-prediction mode for the chroma signal of the current block is not6 or more and 14 or less, whether or not the intra-prediction mode forthe chroma signal of the current block is 22 or more and 30 or less isdetermined at step S1360. If, as a result of the determination, it isdetermined that the intra-prediction mode for the chroma signal of thecurrent block is 22 or more and 30 or less, horizontal scanning isperformed on the chroma signal of the current block at step S1380.

If, as a result of the determination, it is determined that theintra-prediction mode for the chroma signal of the current block is not6 or more and 14 or less and is not 22 or more and 30 or less, up-rightdiagonal scanning is performed on the chroma signal of the current blockat step S1390.

Unlike in the example of FIG. 12, in the example of FIG. 13, anintra-prediction mode for the chroma signal of the current block isdetermined and a scan type is determined based on a result of thedetermination. Like in the example of FIG. 12, however, a scanningmethod for a chroma signal and a luma signal is determined as the samemethod.

In other words, the flowchart of FIG. 13 can be equally applied to thescanning of a luma signal. The reason why IntraPredMode(C) is indicatedusing parentheses at steps S1350 and S1360 is that an intra-predictionmode for the luma signal can be specified by IntraPredMode and appliedand an intra-prediction mode for the chroma signal can be specified byIntraPredModeC and applied.

Accordingly, the flowchart of FIG. 13 can be equally applied to a lumasignal and a chroma signal, and a scanning type for the luma signal anda scanning type for the chroma signal can be determined as the samemethod. Here, the scanning type is any one of vertical scanning,horizontal scanning, and up-right diagonal scanning.

Table 4 is obtained by incorporating the examples of FIGS. 12 and 13into a syntax structure for a Transform Unit (hereinafter referred to asa ‘TU’).

Table 4 shows an example of the TU syntax according to the presentembodiment 1.

TABLE 4 transform_unit( x0, y0, xC, vC, 1og2TrafoWidth, log2TrafoHeight,trafoDepth, blkIdx ) { Descriptor ...  1og2TrafoSize = ( (log2TrafoWidth +  1og2TrafoHeight ) >> 1)  1og2TrafoSizeC = ( (log2TrafoSize = =  Log2MinTrafoSizeC ) ?  1og2TrafoSize : 1og2TrafoSize− 1 )  if( PredMode = = MODE_INTRA ) {   scanIdx = ScanType[log2TrafoSize − 2 ]   [ IntraPredMode ]   scanIdxC = scanIdx  } else {  scanIdx = 0   scanIdxC = 0  } ...

In Table 4, transform unit indicates order of bit streams for one TUblock. log2TrafoSize indicates a result of right shift operation for thesum of log2TrafoWidth and log2TrafoHeight and indicates the size of a TUblock for a luma signal. log2TrafoSizeC indicates the size of a TU blockfor a chroma signal.

PredMode indicates an encoding mode for a current block. PredMode is‘Intra’ in the case of intra-frame encoding and is ‘Inter’ in the caseof inter-frame encoding. scanIdx indicates information about a scanningdirection for the luma signal of a current TU block and includes anup-right direction DIAG (Up-right=0), a horizontal direction HOR(Horizontal=1), and a vertical direction VER (Vertical=2).

scanIdxC indicates information about a scanning direction for the chromasignal of a current TU block and includes an up-right direction DIAG(Up-right=0), a horizontal direction HOR (Horizontal=1), and a verticaldirection VER (Vertical=2).

ScanType is determined based on the table of a method of determining ascanning direction according to a frame prediction direction of Table 2.

IntraPredMode indicates information about an intra-prediction directionfor a luma signal. IntraPredModeC indicates information about anintra-prediction direction for a chroma signal.

In relation to Table 4, a transform coefficient level can be parsed in atransCoeffLevel array. Here, if PredMode is Intra, a different scanningdirection is applied depending on an intra-prediction direction mode.This scanning direction can be obtained from the ScanType array in Table2.

A scanning direction for a luma signal can be used as a scanningdirection for a chroma signal without change.

[Embodiment 2-1] A method and apparatus for applying a scanningdirection for a luma signal to a chroma signal without change when anintra-prediction direction mode for the chroma signal is‘Intra_FromLuma’

FIG. 14 is a flowchart illustrating an embodiment of a method ofderiving a scanning direction for a chroma signal according to thepresent embodiment.

Referring to FIG. 14, whether or not an intra-prediction direction modefor the chroma signal of a current block is an intra-prediction mode isdetermined at step S1410. If, as a result of the determination, it isdetermined that an intra-prediction mode has not been applied to thecurrent block, up-right diagonal scanning is performed on the currentblock at step S1480.

If, as a result of the determination, it is determined that the currentblock has been subjected to intra-prediction, whether or not theintra-prediction mode for the chroma signal of the current block isIntra_FromLuma is determined at step S1420. If, as a result of thedetermination, it is determined that the intra-prediction mode for thechroma signal of the current block is Intra_FromLuma, anintra-prediction mode for the luma signal of the current block is set asthe intra-prediction mode for the chroma signal of the current block atstep S1440.

In other words, if the intra-prediction mode for the chroma signal ofthe current block is Intra_FromLuma, a scanning direction for the chromasignal can be derived by inputting the intra-prediction direction modeof the luma signal to the intra-prediction direction mode of the chromasignal. That is, if the intra-prediction direction mode of the chromasignal is Intra_FromLuma, the scanning direction for the luma signal canbe used as the scanning direction for the chroma signal.

Next, whether or not the intra-prediction mode for the chroma signal ofthe current block is 6 or more and 14 or less is determined at stepS1430. If, as a result of the determination, it is determined that theintra-prediction mode for the chroma signal of the current block is 6 ormore and 14 or less, vertical scanning is performed on the chroma signalof the current block at step S1460.

If, as a result of the determination, it is determined that theintra-prediction mode for the chroma signal of the current block is not6 or more and 14 or less, whether or not the intra-prediction mode forthe chroma signal of the current block is 22 or more and 30 or less isdetermined at step S1450. If, as a result of the determination, it isdetermined that the intra-prediction mode for the chroma signal of thecurrent block is 22 or more and 30 or less, horizontal scanning isperformed on the chroma signal of the current block at step S1470.

If the intra-prediction mode for the chroma signal of the current blockis not 6 or more and 14 or less and is not 22 or more and 30 or less,up-right diagonal scanning is performed on the chroma signal of thecurrent block at step S1480.

[Embodiment 2-2] A method and apparatus for applying a scanningdirection for a luma signal to a chroma signal without change when theintra-prediction direction of the chroma signal is the same as theintra-prediction direction of the luma signal or an intra-predictionmode for the chroma signal is Intra_FromLuma

FIG. 15 is a flowchart illustrating another embodiment of a method ofderiving a scanning direction for a chroma signal according to thepresent embodiment.

From FIG. 15, it can be seen that a DM method (i.e., a method of usingan intra-prediction direction mode for a luma signal as anintra-prediction direction mode for a chroma signal or it means that theintra-prediction direction of the chroma signal becomes equal to theintra-prediction direction of the luma signal) and the LM(Intra_FromLuma) method are unified into one.

First, whether or not a current block has been subjected tointra-prediction is determined at step S1505. If, as a result of thedetermination, it is determined that an intra-prediction mode has notbeen applied to the current block, up-right diagonal scanning isperformed on the current block at step S1575.

If, as a result of the determination, it is determined that the currentblock has been subjected to intra-prediction, the width of the currentblock is changed into an index at step S1510.

Whether or not a target sample to be determined is a luma signal isdetermined at step S1515. If, as a result of the determination, it isdetermined that the target sample is not a luma signal, whether or notthe DM or LM is applied to the current block is determined at stepS1530. If, as a result of the determination, it is determined that thetarget sample is a luma signal, information about an intra-predictionmode is obtained at step S1520.

If the present invention is limitedly applied to a case whereintra-frame encoding or decoding is performed on a chroma signal usingthe DM without using the LM for the purpose of profiling depending on anapplication field, whether or not the DM is applied without determiningwhether or not the LM is applied may be determined at step S1530 in theexample of FIG. 15.

After obtaining the information about an intra-prediction mode, if theindex is greater than 3 and smaller than 6 (i.e., a transform size is8×8 or 4×4) at step S1525, whether or not an intra-prediction mode forthe luma signal of the current block is 6 or more and 14 or less isdetermined at step S1555. If, as a result of the determination, it isdetermined that the intra-prediction mode for the luma signal of thecurrent block is 6 or more and 14 or less, vertical scanning isperformed on the luma signal of the current block at step S1565.

If, as a result of the determination, it is determined that theintra-prediction mode for the luma signal of the current block is not 6or more and 14 or less, whether or not the intra-prediction mode for theluma signal of the current block is 22 or more and 30 or less isdetermined at step S1560. If, as a result of the determination, it isdetermined that the intra-prediction mode for the luma signal of thecurrent block is 22 or more and 30 or less, horizontal scanning isperformed on the current block at step S1570. If, as a result of thedetermination, it is determined that the intra-prediction mode for theluma signal of the current block is not 22 or more and 30 or less,up-right diagonal scanning is performed on the current block at stepS1575.

Meanwhile, if, as a result of the determination at step S1515, it isdetermined that the target sample is a chroma signal not a luma signal,whether or not a mode for the current block is the DM or LM isdetermined at step S1530. If, as a result of the determination, it isdetermined that the mode for the chroma signal of the current block isnot the DM or LM, information about an intra-prediction mode for thechroma signal is obtained at step S1545. If, as a result of thedetermination, it is determined that the mode for the chroma signal ofthe current block is the DM or LM, information about an intra-predictionmode for the luma signal is obtained at step S1535. In order todetermine a scanning direction for the chroma signal, theintra-prediction mode for the chroma signal is set as anintra-prediction mode for the luma signal at step S1540.

Next, whether or not the index is greater than 4 and smaller than 7(i.e., whether a transform size is 4×4 or 2×2) is determined at stepS1550. If, as a result of the determination, it is determined that theindex is not greater than 4 and smaller than 7, up-right diagonalscanning is performed on the chroma signal of the current block at stepS1575.

If, as a result of the determination, it is determined that the index isgreater than 4 and smaller than 7, whether or not the intra-predictionmode for the chroma signal of the current block is 6 or more and 14 orless is determined at step S1555. If, as a result of the determination,it is determined that the intra-prediction mode for the chroma signal ofthe current block is 6 or more and 14 or less, vertical scanning isperformed on the chroma signal of the current block at step S1565.

If, as a result of the determination, it is determined that theintra-prediction mode for the chroma signal of the current block is not6 or more and 14 or less, whether or not the intra-prediction mode forthe chroma signal of the current block is 22 or more and 30 or less isdetermined at step S1560. If, as a result of the determination, it isdetermined that the intra-prediction mode for the luma signal of thecurrent block is 22 or more and 30 or less, horizontal scanning isperformed on the current block at step S1570. If, as a result of thedetermination, it is determined that the intra-prediction mode for theluma signal of the current block is not 22 or more and 30 or less,up-right diagonal scanning is performed on the current block at stepS1575.

Table 5 is obtained by incorporating the examples of FIGS. 14 and 15into a syntax structure for a Transform Unit (hereinafter referred to asa ‘TU’).

Table 5 shows an example of the TU syntax according to the presentembodiment 1.

TABLE 5 transform_unit( x0, y0, xC, vC, 1og2TrafoWidth, log2TrafoHeight,trafoDepth, blkIdx ) { Descriptor ...  1og2TrafoSize = ( (log2TrafoWidth +  1og2TrafoHeight ) >> 1)  1og2TrafoSizeC = ( (log2TrafoSize = =  Log2MinTrafoSizeC ) ?  1og2TrafoSize : 1og2TrafoSize− 1)  if( PredMode = = MODE_INTRA ) {   scanIdx = ScanType[log2TrafoSize − 2 ]   [ IntraPredMode ]   scanIdxC = (IntraPredModeC = =Intra_  FromLuma) ? scanIdx :  ScanType[ log2TrafoSize − 2][IntraPredModeC ]  } else {   scanIdx = 0   scanldxC = 0  } ...

In Table 5, transform unit indicates order of bit streams for one TUblock. log2TrafoSize indicates a result of right shift operation for thesum of log2TrafoWidth and log2TrafoHeight and indicates the size of a TUblock for a luma signal.

log2TrafoSizeC indicates the size of a TU block for a chroma signal.PredMode indicates an encoding mode for a current block. PredMode is‘Intra’ in the case of intra-frame encoding and is ‘Inter’ in the caseof inter-frame encoding.

scanIdx indicates information about a scanning direction for the lumasignal of a current TU block and includes an up-right direction DIAG(Up-right=0), a horizontal direction HOR (Horizontal=1), and a verticaldirection VER (Vertical=2).

scanIdxC indicates information about a scanning direction for the chromasignal of a current TU block and includes an up-right direction DIAG(Up-right=0), a horizontal direction HOR (Horizontal=1), and a verticaldirection VER (Vertical=2).

ScanType is determined based on the table of a method of determining ascanning direction according to a frame prediction direction of Table 2.

IntraPredMode indicates information about an intra-prediction directionfor a luma signal. IntraPredModeC indicates information about anintra-prediction direction for a chroma signal. As indicated in thesyntax table, if a value of IntraPredModeC is Intra_FromLuma, a scanningdirection used in a luma signal is applied to a chroma signal withoutchange.

In relation to Table 5, a transform coefficient level can be parsed in atransCoeffLevel array. Here, if PredMode is Intra, a different scanningdirection is applied depending on an intra-prediction direction mode.This scanning direction can be obtained from the ScanType array in Table6 below. In order to obtain scanIdx, that is, a scanning directionindicator for a luma signal, a value of IntraPredMode indicative of anintra-prediction mode for the luma signal is used as a value ofIntraPredModeValue in Table 6. In order to obtain scanIdxC, that is, ascanning direction indicator for a chroma signal, a value IntraPredModeCindicative of an intra-prediction mode for the chroma signal is used asa value of IntraPredModeValue in Table 6.

Table 6 shows an example of a method of determining a scan type.

TABLE 6 IntraPred- log2TrafoSize-2 ModeValue 0 1 2 3  0 0 0 0 0  1 0 0 00  2-5 0 0 0 0  6-14 2 2 0 0 15-21 0 0 0 0 22-30 1 1 0 0 31-34 0 0 0 0

In Table 6, a scan type ScanType can be determined by log2TrafoSize andIntraPredModeValue as follows:ScanType[log2TrafoSize-2][IntraPredModeValue]

As described above, a transform coefficient level is parsed in atransCoeffLevel array. Here, if PredMode is ‘Intra’, a differentscanning direction is used depending on an intra-prediction directionmode.

Here, ScanType[log2TrafoSize-2][IntraPredModeValue] can be set accordingto a syntax structure of Table 7 based on the size ‘log2TrafoSize’ andan intra-prediction direction mode ‘IntraPredModeValue’ of a luma signaltransform block.

TABLE 7 - If ( log2TrafoSize == 2 || log2TrafoSize == 3 ) the followingapplies.  - If ( IntraPredModeValue >= 22 && IntraPredModeValue <= 30 ), ScanType[ log2TrafoSize − 2 ][ IntraPredModeValue ] is set to 1.  -Otherwise, if ( IntraPredModeValue >= 6 && IntraPredModeValue <=  10 ),ScanType[ log2TrafoSize − 2 ][ IntraPredModeValue ] is set to 2.  -Otherwise, ScanType[ log2TrafoSize − 2 ][ IntraPredModeValue ] is set to 0. - Otherwise, ScanType[ log2TrafoSize − 2 ][ IntraPredModeValue ] isset to 0.

In order to obtain scanIdx, that is, a scanning direction indicator fora luma signal, a value of IntraPredMode indicative of anintra-prediction mode for the luma signal is used as a value ofIntraPredModeValue in Table 2. In order to obtain scanIdxC, that is, ascanning direction indicator for a chroma signal, a value ofIntraPredModeC indicative of an intra-prediction mode for the chromasignal is used as a value of IntraPredModeValue in Table 2.

The present embodiment can also be applied to the following cases inaddition to a case where a current chroma block has been encoded inIntra_FromLuma (LM).

1. In predicting a chroma block, the present embodiment can be appliedto all methods of multiplying a reconstructed luma signal block byweight and adding an offset to a result of the multiplication.

2. In predicting a chroma block, the present embodiment can be appliedto all methods of predicting a chroma block using a template shift forreconstructed luma signal and chroma signal blocks. Here, the templateshift is used to derive a correlation between a luma signal and a chromasignal.

In addition, the present embodiment can be applied in various ways.

For example, the present embodiment can be differently applied to anapplication scope depending on the size of all blocks, a CU depth, or aTU depth. A parameter (i.e., information about the size or depth) usedto determine an application scope as described above may be set so thata predetermined value is used in an encoder and a decoder, or apredetermined value may be used depending on a profile or level. Forexample, when an encoder writes a parameter value in a bit stream, adecoder may obtain a value from the bit stream and use the value.

If the application scope is different depending on a CU depth, one ofthe following 3 methods can be used as illustrated in Table 8: A methodA) of applying the application scope to only a depth equal to or greaterthan a specific depth, a method B) of applying the application scope toonly a depth equal to or smaller than a specific depth, and a method C)of applying the application scope to only a specific depth.

Table 8 shows an example of methods of determining the application scopein which each of the three methods according to the present embodimentis applied when a specific CU (or TU) depth is 2. In Table 8, ‘O’indicates that the application scope is applied to a correspondingdepth, and ‘X’ indicates that the application scope is not applied to acorresponding depth.

TABLE 8 CU (OR TU) DEPTH INDICATING APPLICATION SCOPE METHOD A METHOD BMETHOD C 0 X O X 1 X O X 2 O O 0 3 O X X 4 O X X

If the methods of the present invention are not applied to all depths,it may be indicated using a specific indicator (or flag) or may berepresented by signaling a value that is 1 greater than a maximum valueof a CU depth using as a CU depth value indicative of the applicationscope.

Furthermore, it is to be noted that a method of determining anapplication scope in which the methods of the present invention areapplied can be applied to the aforementioned embodiment 1 andembodiments to be described later individually or in combination.

[Embodiment 3] A method and apparatus for selectively deriving afrequency transform method for the residual picture of a chroma signalbased on an intra-prediction direction mode for the chroma signal

In the present embodiment, encoding efficiency can be improved byapplying a selective frequency transform method according to anintra-prediction direction mode to which the residual picture of a lumasignal has been applied to the residual picture of a chroma signal.

Furthermore, in the present embodiment, frequency transform can beselectively performed based on a property that a residual signal for achroma signal obtained by the Intra_FromLuma method has a similarcharacteristic to the residual signal of a luma signal.

FIG. 16 is a flowchart illustrating an example of a method ofselectively deriving a frequency transform method for the residualpicture of a chroma signal based on an intra-prediction direction modefor a chroma signal according to the present embodiment.

Referring to FIG. 16, first, if a current block has been subjected tointra-frame encoding ‘Intra’ and the current block has a 4×4 size(trafoSize (or iWidth)==4) at step S1605, integer DST or integer DCT isapplied to a frequency transform method for the current block. If thecurrent block does not have a 4×4 size (trafoSize (or iWidth)==4) atstep S1605, DCT can be applied to a frequency transform method for thecurrent block at step S1660.

Next, if the current block is a luma block at step S1610, anintra-prediction direction mode IntraPredMode for a luma signal isobtained at step S1620. Here, the intra-prediction direction mode forthe luma signal of the current block can be used to derive a frequencytransform method for the residual picture of the luma signal.

If the current block is a chroma block at step S1610, whether or not thechroma block is a DM block is determined at step S1615.

If, as a result of the determination, it is determined that the currentblock is a DM block, DCT can be applied to a frequency transform methodfor the current block at step S1660. If, as a result of thedetermination at step S1615, it is determined that the current block isnot a DM block, the intra-prediction direction mode for the chromasignal can be used to derive a frequency transform method for theresidual picture of the chroma signal at step S1625.

Here, if the chroma block has been encoded in DM, DST, DCT, or otherfrequency transform methods can be applied to an intra-predictiondirection mode corresponding to the EM (e.g., a planar mode PLANAR, amean mode DC, a horizontal mode HOR, a vertical mode VER, or a modelocated at an eighth place in a vertical direction (e.g., Ver+8 or No.34 mode)), from the DMs.

Next, if the intra-prediction direction mode for the current block(i.e., the luma and chroma signal blocks) is 2 or more and 10 or less atstep S1630, integer DST can be applied to a frequency transform methodfor the current block (i.e., the luma and chroma signal blocks) in ahorizontal direction and integer DCT can be applied to a frequencytransform method for the current block (i.e., the luma and the chromasignal blocks) in a vertical direction at step S1645.

In contrast, if the intra-prediction direction mode for the currentblock (i.e., the luma and chroma signal blocks) is 0 (PLANAR) or is 11or more and 25 or less at step S1635, integer DST can be applied to afrequency transform method for the current block (i.e., the luma andchroma signal blocks) in both horizontal and vertical directions at stepS1650.

In contrast, if the intra-prediction direction mode for the currentblock (i.e., the luma and chroma signal blocks) is 26 or more and 34 orless at step S1640, integer DCT can be applied to a frequency transformmethod for the current block (i.e., the luma and chroma signal blocks)in a horizontal direction and integer DST can be applied to a frequencytransform method for the current block (i.e., the luma and chroma signalblocks) in a vertical direction at step S1655.

If the intra-prediction direction mode for the current block (i.e., theluma and chroma signal blocks) is not 26 or more and 34 or less at stepS1640, integer DCT can be applied to a frequency transform method forthe current block (i.e., the luma and chroma signal blocks) in bothhorizontal and vertical directions at step S1660.

The method of FIG. 16 can be incorporated into a transform process forscaled transform coefficients as follows.

Transform Process for Scaled Transform Coefficients into which theExample of FIG. 16 has Been Incorporated

In this case, the input is as follows.

-   -   The width of a current transform block: nW    -   The height of the current transform block: nH    -   An array of scaled transform coefficients having an element dij:        (nWxnH) array d

An index for the luma signal and the chroma signal of a current block:cIdx

If cIdx is 0, it means a luma signal. If cIdx is 1 or cIdx is 2, itmeans a chroma signal. Furthermore, if cIdx is 1, it means Cb in achroma signal. If cIdx is 2, it means Cr in a chroma signal.

In this case, the output is as follows.

-   -   An array for a residual signal obtained by inversely        transforming the scaled transform coefficients: (nWxnH) array r

If an encoding mode ‘PredMode’ for the current block is anintra-prediction mode ‘Intra’, a value of Log2(nW*nH) is 4, and a signalis a luma signal, parameters horizTrType and vertTrType according toTable 9 are used depending on intra-prediction direction modes for theluma signal.

In the case of a chroma signal, if an intra-prediction encoding mode forthe chroma signal is smaller than 4, the parameters horizTrType andvertTrType according to Table 9 are used depending on intra-predictiondirection modes for the chroma signal. If not, the parametershorizTrType and vertTrType are set to 0.

If an intra-prediction direction mode for the chroma signal is ‘35’, theparameters horizTrType and vertTrType are set to 0.

TABLE 9 IntraPredMode 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17vertTrType 1 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 horizTrType 1 0 1 1 1 1 11 1 1 1 1 1 1 1 1 1 1 IntraPredMode 18 19 20 21 22 23 24 25 26 27 28 2930 31 32 33 34 vertTrType 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 horizTrType1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0

An inverse transform process is performed on the scaled transformcoefficients using the parameters horizTrType and vertTrType value.First, the sizes nW and nH and scaled transform coefficients array‘nWxnH array d’ of the current block and the parameter horizTrType arereceived, 1-D inverse transform is performed in a horizontal direction,and an array ‘nWxnH array e’ is output as a result of the 1-D inversetransform.

Next, an array ‘nWxnH array e’ is received, and an array ‘nWxnH array g’is derived as in Equation 5 using the array.

g _(ij)=Clip3(−32768,32767,(e _(uij)+64)>>7)   <Equation 5>

Next, the sizes nW and nH and array ‘nWxnH array g’ of the current blockand the parameter vertTrType are received, and 1-D inverse transform isperformed in a vertical direction.

Next, an array ‘nWxnH array r’ for the residual signals is set based oncIdx as in Equation 6.

r _(ij)=(f _(ij)+(1<<(shift-1)))>>shift   <Equation 6>

Here, when cIdx is 0, shift=20—BitDepthy. When cIdx is not 0,shift=20-BitDepthc. BitDepth indicates the number of bits (e.g., 8 bits)of a sample for a current picture.

FIG. 17 is a flowchart illustrating another example of a method ofselectively deriving a frequency transform method for the residualpicture of a chroma signal based on an intra-prediction direction modefor the chroma signal according to the present embodiment.

Referring to FIG. 17, first, whether or not a current block has beensubjected to intra-frame encoding ‘Intra’ is determined at step S1710.If, as a result of the determination, it is determined that the currentblock has been subjected to intra-frame encoding, integer DST or integerDCT is applied to a frequency transform method for the current block.

If, as a result of the determination, it is determined that the currentblock has not been subjected to intra-frame encoding ‘Intra’, anintra-prediction direction mode ‘IntraPredMode(C)’ for the luma orchroma signal of the current block is obtained at step S1715. Here, whenderiving a frequency transform method for the luma signal,IntraPredMode(C) is changed into IntraPredMode (i.e., theintra-prediction direction mode for the luma signal). When deriving afrequency transform method for the chroma signal, IntraPredMode(C) ischanged into IntraPredModeC (i.e., the intra-prediction direction modefor the chroma signal). Here, the intra-prediction direction mode forthe luma signal of the current block can be used to derive a frequencytransform method for the residual picture of the luma signal, and theintra-prediction direction mode for the chroma signal of the currentblock can be used to derive a frequency transform method for theresidual picture of the chroma signal.

Here, an intra-prediction direction mode for selecting a frequencytransform method can be obtained in various ways as in one of thefollowing 1 to 3 methods.

1. An intra-prediction direction mode for the luma signal of a currentblock can be used to derive a frequency transform method by applying theintra-prediction direction mode to both the residual pictures of theluma signal and a chroma signal.

2. An intra-prediction direction mode for the chroma signal of a currentblock can be used to derive a frequency transform method by applying theintra-prediction direction mode to both the residual pictures of a lumasignal and the chroma signal.

3. Only integer DCT or integer DST can be used in the luma signal of acurrent block. Furthermore, an intra-prediction direction mode for thechroma signal of the current block can be used to derive a frequencytransform method for the residual picture of the chroma signal.

In addition, a frequency transform method can be derived in variousways.

Next, whether or not the current block has a 4×4 size (iWidth==4) isdetermined at step S1720. If, as a result of the determination, it isdetermined that the current block does not have a 4×4 size (iWidth==4),integer transform or integer DCT is applied to a frequency transformmethod for the current block at step S1755.

If, as a result of the determination, it is determined that the currentblock has a 4×4 size (iWidth==4), an intra-prediction direction mode forthe current block is checked.

If the intra-prediction direction mode of the current block is 2 or moreand 10 or less at step S51725, integer DST can be applied to a frequencytransform method for the current block in a horizontal direction andinteger DCT can be applied to a frequency transform method for thecurrent block in a vertical direction at step S1740.

In contrast, if the intra-prediction direction mode of the current blockis 0 or is 11 or more and 25 or less at step S1730, integer DST can beapplied to a frequency transform method for the current block in bothhorizontal and vertical directions at step S1745.

In contrast, if the intra-prediction direction mode of the current blockis 26 or more and 34 or less at step S1735, integer DCT can be appliedto a frequency transform method for the current block in a horizontaldirection and integer DST can be applied to a frequency transform methodfor the current block in a vertical direction at step S1750.

If the intra-prediction direction mode of the current block is not 26 ormore and 34 or less at step S1735, integer DCT can be applied to afrequency transform method for the current block in both horizontal andvertical directions at step S1755.

Here, a frequency transform method according to an intra-predictiondirection mode can be used in various ways. For example, if anintra-prediction direction mode for a current block is 2 or more and 10or less, a frequency transform method may be applied as in theabove-described example. As another method, integer DCT can be appliedto a frequency transform method for the current block in a horizontaldirection and integer DST can be applied to a frequency transform methodfor the current block in a vertical direction. As yet another method,integer DCT can be applied to a frequency transform method for thecurrent block in both horizontal and vertical directions, or integer DSTcan be applied to a frequency transform method for the current block inboth horizontal and vertical directions. Furthermore, integer DST may beapplied all horizontal and vertical directions other than a DC mode(i.e., mode not a mean value prediction and direction mode).

A method of applying various frequency transform methods in various waysas described above can be applied to other intra-prediction directionmode scopes in addition to a case where an intra-prediction directionmode for a current block is 2 or more and 10 or less. The followingcontents can be applied to the following embodiments in the same manner.

The method of FIG. 17 can be incorporated into a transform process forscaled transform coefficients as follows.

Transform Process for Scaled Transform Coefficients into Which theExample of FIG. 17 has Been Incorporated

In this case, the input is as follows.

-   -   The width of a current transform block: nW    -   The height of the current transform block: nH    -   An array of scaled transform coefficients having an element dij:        (nWxnH) array d

An index for the luma signal and the chroma signal of a current block:cIdx

-   -   If cIdx is 0, it means a luma signal. If cIdx is 1 or cIdx is 2,        it means a chroma signal. Furthermore, if cIdx is 1, it means Cb        in a chroma signal. If cIdx is 2, it means Cr in a chroma        signal.

In this case, the output is as follows.

An array for a residual signal obtained by inversely transforming thescaled transform coefficients: (nWxnH) array r

If an encoding mode ‘PredMode’ for the current block is anintra-prediction mode ‘Intra’ and a value of Log2(nW*nH) is 4, (1)parameters horizTrType and vertTrType according to Table 10 are useddepending on intra-prediction direction modes for a luma signal and (2)the parameters horizTrType and vertTrType according to Table 10 are useddepending on intra-prediction direction modes for a chroma signal. Ifnot, the parameters horizTrType and vertTrType are set to 0.

TABLE 10 IntraPredMode 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17vertTrType 1 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 horizTrType 1 0 1 1 1 1 11 1 1 1 1 1 1 1 1 1 1 IntraPredMode 18 19 20 21 22 23 24 25 26 27 28 2930 31 32 33 34 vertTrType 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 horizTrType1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0

An inverse transform process is performed on the scaled transformcoefficients using the parameters horizTrType and vertTrType value.First, the sizes nW and nH and scaled transform coefficients array‘nWxnH array d’ of the current block and the parameter horizTrType arereceived, 1-D inverse transform is performed in a horizontal direction,and an array ‘nWxnH array e’ is output as a result of the 1-D inversetransform.

Next, an array ‘nWxnH array e’ is received, and an array ‘nWxnH array g’is derived as in Equation 7 using the array.

g _(ij)=Clip3(−32768,32767,(e _(ij)+64)>>7) <Equation 7>

Next, the sizes nW and nH and array ‘nWxnH array g’ of the current blockand the parameter vertTrType are received, and 1-D inverse transform isperformed in a vertical direction.

Next, an array ‘nWxnH array r’ for the residual signals is set based oncIdx as in Equation 8.

r _(ij)=(f _(ij)+(1<<(shift-1)))>>shift   <Equation 8>

Here, when cIdx is 0, shift=20—BitDepthy. When cIdx is not 0,shift=20—BitDepthc. BitDepth indicates the number of bits (e.g., 8 bits)of a sample for a current picture.

[Embodiment 4] A method and apparatus for selectively deriving afrequency transform method for the residual picture of a chroma signalbased on an intra-prediction direction mode for a luma signal inrelation to an Intra_FromLuma (LM) encoding mode for the chroma signal

FIG. 18 is a flowchart illustrating an example of a method ofselectively deriving a frequency transform method for the residualpicture of a chroma signal based on an intra-prediction direction modefor a luma signal in relation to the LM of the chroma signal accordingto the present embodiment.

Referring to FIG. 18, first, if a current block has been subjected tointra-frame encoding ‘Intra’ and the current block has a 4×4 size(TrafoSize (or iWidth) 4) at step S1810, integer DST or integer DCT isapplied to a frequency transform method for the current block at stepS1855. If the current block does not have a 4×4 size (trafoSize (oriWidth)==4) at step S1810, DCT can be applied to a frequency transformmethod for the current block.

Next, if the current block is a luma block or a chroma block and anintra-prediction direction mode for the chroma block is the LM at stepS1815, an intra-prediction direction mode ‘IntraPredMode’ for the lumasignal is obtained at step S1820.

Here, the intra-prediction direction mode for the luma signal of thecurrent block can be used to derive a frequency transform method for theresidual pictures of the luma signal and the chroma signal.

If the current block is a chroma block (i.e., a chroma block not the LM)at step S1815, DCT can be applied to a frequency transform method forthe current block at step S1855.

Next, if the intra-prediction direction mode for the current block(i.e., the luma and chroma signal blocks) is 2 or more and 10 or less atstep S1825, integer DST can be applied to a frequency transform methodfor the current block (i.e., the luma and chroma signal blocks) in ahorizontal direction and integer DCT can be applied to a frequencytransform method for the current block (i.e., the luma and chroma signalblocks) in a vertical direction at step S1840.

In contrast, if the intra-prediction direction mode for the currentblock (i.e., the luma and chroma signal blocks) is 0 (PLANAR) or is 11or more and 25 or less at step S1830), integer DST can be applied to afrequency transform method for the current block (i.e., the luma andchroma signal blocks) in both horizontal and vertical directions at stepS1845.

In contrast, if the intra-prediction direction mode for the currentblock (i.e., the luma and chroma signal blocks) is 26 or more and 34 orless at step S1835, integer DCT can be applied to a frequency transformmethod for the current block (i.e., the luma and chroma signal blocks)in a horizontal direction and integer DST can be applied to a frequencytransform method for the current block (i.e., the luma and chroma signalblocks) in a vertical direction at step S1850. If the intra-predictiondirection mode for the current block (i.e., the luma and chroma signalblocks) is not 26 or more and 34 or less at step S1835, integer DCT canbe applied to a frequency transform method for the current block (i.e.,the luma and chroma signal blocks) in both horizontal and verticaldirections at step S1855.

Meanwhile, a transform process for scaled transform coefficientsaccording to the example of FIG. 18 is as follows.

A transform Process for Scaled Transform Coefficients According to theExample of FIG. 18

In this case, the input is as follows.

-   -   The width of a current transform block: nW    -   The height of the current transform block: nH    -   An array of scaled transform coefficients having an element dij:        (nWxnH) array d

An index for the luma signal and the chroma signal of a current block:cIdx

-   -   If cIdx is 0, it means a luma signal. If cIdx is 1 or cIdx is 2,        it means a chroma signal. Furthermore, if cIdx is 1, it means Cb        in a chroma signal. If cIdx is 2, it means Cr in a chroma        signal.

In this case, the output is as follows.

An array for a residual signal obtained by inversely transforming thescaled transform coefficients: (nWxnH) array r

If an encoding mode ‘PredMode’ for the current block is anintra-prediction mode ‘Intra’ and a value of Log2(nW*nH) is 4,parameters horizTrType and vertTrType according to Table 11 are useddepending on intra-prediction direction modes for a luma signal and theparameters horizTrType and vertTrType according to Table 11 are useddepending on intra-prediction direction modes for the chroma signal ifan encoding mode for the chroma signal is Intra_FromLuma (LM). If not,the parameters horizTrType and vertTrType are set to 0.

TABLE 11 IntraPredMode 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17vertTrType 1 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 horizTrType 1 0 1 1 1 1 11 1 1 1 1 1 1 1 1 1 1 IntraPredMode 18 19 20 21 22 23 24 25 26 27 28 2930 31 32 33 34 vertTrType 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 horizTrType1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0

An inverse transform process is performed on the scaled transformcoefficients using the parameters horizTrType and vertTrType value.First, the sizes nW and nH and scaled transform coefficients array‘nWxnH array d’ of the current block and the parameter horizTrType arereceived, 1-D inverse transform is performed in a horizontal direction,and an array ‘nWxnH array e’ is output as a result of the 1-D inversetransform.

Next, an array ‘nWxnH array e’ is received, and an array ‘nWxnH array g’is derived as in Equation 9 using the array.

g _(ij)=Clip3(−32768,32767,(e_(ij)+64)>>7) <Equation 9>

Next, the sizes nW and nH and array ‘nWxnH array g’ of the current blockand the parameter vertTrType are received, and 1-D inverse transform isperformed in a vertical direction.

Next, an array ‘nWxnH array r’ for the residual signals is set based oncIdx as in Equation 10.

r _(ij)=(f _(ij)+(1<<(shift-1)))>>shift <Equation 10>

Here, when cIdx is 0, shift=20—BitDepthy. When cIdx is not 0,shift=20—BitDepthc. BitDepth indicates the number of bits (e.g., 8 bits)of a sample for a current picture.

FIG. 19 is a flowchart illustrating another example of a method ofselectively deriving a frequency transform method for the residualpicture of a chroma signal based on an intra-prediction direction modefor a luma signal in relation to an Intra_FromLuma (LM) encoding modefor the chroma signal according to the present embodiment.

Referring to FIG. 19, whether or not a current block has been encodedaccording to Condition 1 (i.e., whether or not the current block hasbeen subjected to intra-frame encoding ‘Intra’ and the current block isthe block of a luma signal) or Condition 2 (i.e., whether or not thecurrent block has the LM and the current block is the block of a chromasignal) is determined at step S1910.

If, as a result of the determination, Condition 1 or Condition 2 is notsatisfied, integer transform or integer DCT can be applied to frequencytransform methods for the luma signal and the chroma signal of thecurrent block at step S1955.

If, as a result of the determination, Condition 1 or Condition 2 issatisfied, an intra-prediction direction mode ‘IntraPredMode’ for theluma signal of the current block is obtained at step S1915.

Here, an intra-prediction direction mode for the luma signal of thecurrent block can be used to derive a frequency transform method for theresidual picture of the luma signal. Furthermore, if the chroma signalof the current block has been encoded in the LM, an intra-predictiondirection mode for the luma signal of the current block can be used toderive a frequency transform method for the residual picture of thechroma signal.

Next, whether or not the current block has a 4×4 size (iWidth==4) isdetermined at step S1920. If, as a result of the determination, it isdetermined that the current block does not have a 4×4 size (iWidth==4),integer transform or integer DCT can be applied to frequency transformmethods for the luma signal and the chroma signal of the current blockat step S1955. If, as a result of the determination, it is determinedthat the current block has a 4×4 size (iWidth==4), the intra-predictiondirection mode for the current block is checked.

If the intra-prediction direction mode of the current block is 2 or moreand 10 or less at step S1925, integer DST can be applied to a frequencytransform method for the luma signal of the current block in ahorizontal direction and integer DCT can be applied to a frequencytransform method for the luma signal of the current block in a verticaldirection at step S1940. If the chroma signal of the current block hasbeen encoded in the LM, the frequency transform method for the lumasignal of the current block can be used to perform frequency transformon the residual picture of the chroma signal.

If the chroma signal of the current block has not been encoded in theLM, integer DCT can be performed on the chroma signal of the currentblock in both horizontal and vertical directions at step S1955.

In contrast, if the intra-prediction direction mode of the current blockis 0 or is 11 or more and 25 or less at step S1930, integer DST can beapplied to a frequency transform method for the luma signal of thecurrent block in both horizontal and vertical directions at step S1945.If the chroma signal of the current block has been encoded in the LM,the frequency transform method for the luma signal of the current blockcan be used to perform frequency transform on the residual picture ofthe chroma signal. If the chroma signal of the current block has notbeen encoded in the LM, integer DCT can be performed on the chromasignal of the current block in both horizontal and vertical directionsat step S1955.

In contrast, if the intra-prediction direction mode of the current blockis 26 or more and 34 or less at step S1935, integer DCT can be appliedto a frequency transform method for the luma signal of the current blockin a horizontal direction and integer DST can be applied to a frequencytransform method for the luma signal of the current block in a verticaldirection at step 1950. If the chroma signal of the current block hasbeen encoded in the LM, a frequency transform method for the luma signalof the current block can be used to perform frequency transform on theresidual picture of the chroma signal. If the chroma signal of thecurrent block has not been encoded in the LM, integer DCT can beperformed on the chroma signal of the current block in both horizontaland vertical directions at step S1955.

If the intra-prediction direction mode of the current block is not 26 ormore and 34 or less at step S1935, integer DCT can be applied to afrequency transform method for the residual pictures of the luma signaland the chroma signal of the current block in both horizontal andvertical directions at step S1955.

A transform process for scaled transform coefficients into which theexample of FIG. 19 has been incorporated is as follows.

A Transform Process for Scaled Transform Coefficients into Which theExample of FIG. 19 has Been Incorporated

In this case, the input is as follows.

-   -   The width of a current transform block: nW    -   The height of the current transform block: nH    -   An array of scaled transform coefficients having an element dij:        (nWxnH) array d

An index for the luma signal and the chroma signal of a current block:cIdx

-   -   If cIdx is 0, it means a luma signal. If cIdx is 1 or cIdx is 2,        it means a chroma signal. Furthermore, if cIdx is 1, it means Cb        in a chroma signal. If cIdx is 2, it means Cr in a chroma        signal.

In this case, the output is as follows.

An array for a residual signal obtained by inversely transforming thescaled transform coefficients: (nWxnH) array r

If an encoding mode ‘PredMode’ for the current block is anintra-prediction mode ‘Intra’ and a value of Log2(nW*nH) is 4,parameters horizTrType and vertTrType according to Table 12 are useddepending on intra-prediction direction modes for a luma signal and theparameters horizTrType and vertTrType according to Table 12 are useddepending on intra-prediction direction modes for the chroma signal ifan encoding mode for the chroma signal is Intra_FromLuma (LM). If not,the parameters horizTrType and vertTrType are set to 0.

TABLE 12 IntraPredMode 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17vertTrType 1 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 horizTrType 1 0 1 1 1 1 11 1 1 1 1 1 1 1 1 1 1 IntraPredMode 18 19 20 21 22 23 24 25 26 27 28 2930 31 32 33 34 vertTrType 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 horizTrType1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0

An inverse transform process is performed on the scaled transformcoefficients using the parameters horizTrType and vertTrType value.First, the sizes nW and nH and scaled transform coefficients array‘nWxnH array d’ of the current block and the parameter horizTrType arereceived, 1-D inverse transform is performed in a horizontal direction,and an array ‘nWxnH array e’ is output as a result of the 1-D inversetransform.

Next, an array ‘nWxnH array e’ is received, and an array ‘nWxnH array g’is derived as in Equation 11 using the array.

g _(ij)=Clip3(−32768,32767,(e _(ij)+64)>>7) <Equation 11>

Next, the sizes nW and nH and array ‘nWxnH array g’ of the current blockand the parameter vertTrType are received, and 1-D inverse transform isperformed in a vertical direction.

Next, an array ‘nWxnH array r’ for the residual signals is set based oncIdx as in Equation 12.

r _(ij)=(f _(ij)(1<<(shift-1)))>>shift   <Equation 13>

Here, when cIdx is 0, shift=20—BitDepthy. When cIdx is not 0,shift=20—BitDepthc. BitDepth indicates the number of bits (e.g., 8 bits)of a sample for a current picture.

The present embodiment can also be applied to various cases, such as 1and 2 below, in addition to a case where a current chroma block has beenencoded in Intra_FromLuma (LM).

1. In predicting a chroma block, the present embodiment can be appliedto all methods of multiplying a reconstructed luma signal block byweight and adding an offset to a result of the multiplication.

2. In predicting a chroma block, the present embodiment can be appliedto all methods of predicting a chroma block using a template shift forreconstructed luma signal and chroma signal blocks. Here, the templateshift is used to derive a correlation between a luma signal and a chromasignal.

In addition, the present embodiment can be applied in various ways.

[Embodiment 5] A method and apparatus for selectively deriving afrequency transform method for the residual picture of a chroma signaldepending on an intra-prediction direction mode

FIG. 20 is a flowchart illustrating an example of a method ofselectively deriving a frequency transform method for the residualpicture of a chroma signal based on an intra-prediction direction modeaccording to the present embodiment.

Referring to FIG. 20, first, a current block has been subjected tointra-frame encoding ‘Intra’, and whether or not the current block has a4×4 size (trafoSize (or iWidth)==4) is determined at step S2010. First,if the current block has been subjected to intra-frame encoding ‘Intra’and the current block has a 4×4 size (trafoSize (or iWidth)==4), integerDST or integer DCT can be applied to a frequency transform method forthe current block. If the current block does not have a 4×4 size(trafoSize (or iWidth)==4), DCT can be applied to a frequency transformmethod for the current block at step S2065.

Next, if the current block is a luma block or a chroma block and anintra-prediction direction mode for the chroma block is the LM at stepS2015, an intra-prediction direction mode ‘IntraPredMode’ for the lumasignal is obtained at step S2020. Here, the intra-prediction directionmode for the luma signal of the current block can be used to derive afrequency transform method for the residual pictures of the luma signaland the chroma signal.

If an intra-prediction direction mode for the chroma block is not the LMat step S2015, whether or not the chroma block is the DM is determinedat step S2025.

If the current block is the chroma block and is a DM block, DCT can beapplied to a frequency transform method for the current block at stepS2065. If the current block is the chroma block, but is not a DM block,the intra-prediction direction mode for the chroma signal can be used toderive a frequency transform method for the residual picture of thechroma signal at step S2030.

Here, if the chroma block has been encoded in DM, DST, DCT, or otherfrequency transform methods can be applied to an intra-predictiondirection mode corresponding to the EM (e.g., a planar mode PLANAR, amean mode DC, a horizontal mode HOR, a vertical mode VER, or a modelocated at an eighth place in a vertical direction (e.g., Ver+8 or No.34 mode)), from the DMs.

Next, if the intra-prediction direction mode for the current block(i.e., the luma and chroma signal blocks) is 2 or more and 10 or less atstep S2035, integer DST can be applied to a frequency transform methodfor the current block (i.e., the luma and chroma signal blocks) in ahorizontal direction and integer DCT can be applied to a frequencytransform method for the current block (i.e., the luma and the chromasignal blocks) in a vertical direction at step S2050.

In contrast, if the intra-prediction direction mode for the currentblock (i.e., the luma and chroma signal blocks) is 0 (PLANAR) or is 11or more and 25 or less at step S2040, integer DST can be applied to afrequency transform method for the current block (i.e., the luma andchroma signal blocks) in both horizontal and vertical directions at stepS2055.

In contrast, if the intra-prediction direction mode for the currentblock (i.e., the luma and chroma signal blocks) is 26 or more and 34 orless at step S2045, integer DCT can be applied to a frequency transformmethod for the current block (i.e., the luma and chroma signal blocks)in a horizontal direction and integer DST can be applied to a frequencytransform method for the current block (i.e., the luma and chroma signalblocks) in a vertical direction at step S2060.

If the intra-prediction direction mode for the current block (i.e., theluma and chroma signal blocks) is not 26 or more and 34 or less at stepS2045, integer DCT can be applied to a frequency transform method forthe current block (i.e., the luma and chroma signal blocks) in bothhorizontal and vertical directions at step S2064.

The method of selectively applying a frequency transform method can beapplied in various ways as in any one of 1 to 3 below.

1. If a chroma block has been encoded in DM, DST, DCT, or otherfrequency transform methods can be applied to only an intra-predictiondirection mode corresponding to the EM (planar mode PLANAR, a mean modeDC, a horizontal mode HOR, a vertical mode VER, or a mode located at aneighth place in a vertical direction (Ver+8 or No. 34 mode)), from amongthe DMs.

2. If a chroma block has been encoded in the LM, DST, DCT, or otherfrequency transform methods can be applied to only an intra-predictiondirection mode corresponding to the EM (planar mode PLANAR, a mean modeDC, a horizontal mode HOR, a vertical mode VER, or a mode located at aneighth place in a vertical direction (Ver+8 or No. 34 mode)), from amongintra-prediction direction modes derived from a luma signal.

3. Only when an intra-prediction direction mode is PLANAR or DC, DST,DCT, or other frequency transform methods can be selectively applied. Inother modes, DST, DCT, or other frequency transform methods can befixedly applied. In another embodiment, the reverse is possible.

In addition, a frequency transform method can be derived in variousways.

A method of performing the transform process for the scaled transformcoefficients by incorporating the method of FIG. 20 is as follows.

A Transform Process for Scaled Transform Coefficients into Which theExample of FIG. 20 has Been Incorporated

In this case, the input is as follows.

-   -   The width of a current transform block: nW    -   The height of the current transform block: nH    -   An array of scaled transform coefficients having an element dij:        (nWxnH) array d

An index for the luma signal and the chroma signal of a current block:cIdx

-   -   If cIdx is 0, it means a luma signal. If cIdx is 1 or cIdx is 2,        it means a chroma signal. Furthermore, if cIdx is 1, it means Cb        in a chroma signal. If cIdx is 2, it means Cr in a chroma        signal.

In this case, the output is as follows.

An array for a residual signal obtained by inversely transforming thescaled transform coefficients: (nWxnH) array r

If an encoding mode ‘PredMode’ for the current block is anintra-prediction mode ‘Intra’ and a value of Log2(nW*nH) is 4,parameters horizTrType and vertTrType according to Table 13 are useddepending on intra-prediction direction modes for a luma signal. In thecase of a chroma signal, if the intra-prediction encoding mode of thechroma signal is smaller than 4, the parameters horizTrType andvertTrType according to Table 13 are used depending on intra-predictiondirection modes for the chroma signal. In contrast, if an encoding modefor the chroma signal is Intra_FromLuma (LM), the parameters horizTrTypeand vertTrType according to Table 13 are used depending onintra-prediction direction modes for a luma signal. If the encoding modefor the chroma signal is not Intra_FromLuma (LM), the parametershorizTrType and vertTrType are set to 0.

TABLE 13 IntraPredMode 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17vertTrType 1 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 horizTrType 1 0 1 1 1 1 11 1 1 1 1 1 1 1 1 1 1 IntraPredMode 18 19 20 21 22 23 24 25 26 27 28 2930 31 32 33 34 vertTrType 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 horizTrType1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0

An inverse transform process is performed on the scaled transformcoefficients using the parameters horizTrType and vertTrType value.First, the sizes nW and nH and scaled transform coefficients array‘nWxnH array d’ of the current block and the parameter horizTrType arereceived, 1-D inverse transform is performed in a horizontal direction,and an array ‘nWxnH array e’ is output as a result of the 1-D inversetransform.

Next, an array ‘nWxnH array e’ is received, and an array ‘nWxnH array g’is derived as in Equation 13 using the array.

g _(ij)=Clip3(−32768,32767,(e _(ij)+64)>>7)   <Equation 13>

Next, the sizes nW and nH and array ‘nWxnH array g’ of the current blockand the parameter vertTrType are received, and 1-D inverse transform isperformed in a vertical direction.

Next, an array ‘nWxnH array r’ for the residual signals is set based oncIdx as in Equation 14.

r _(ij)=(f _(ij)+(1<<(shift-1)))>>shift   <Equation 14>

Here, when cIdx is 0, shift=20—BitDepthy. When cIdx is not 0,shift=20—BitDepthc. BitDepth indicates the number of bits (e.g., 8 bits)of a sample for a current picture.

FIG. 21 is a flowchart illustrating another example of a method ofselectively deriving a frequency transform method for the residualpicture of a chroma signal based on an intra-prediction direction modeaccording to the present embodiment.

Referring to FIG. 21, first, if a current block has not been subjectedto intra-frame encoding ‘Intra’ at step S2105, integer transform orinteger DCT can be applied to frequency transform methods for the lumasignal and the chroma signal of the current block at step S2160.

If the current block has been subjected to intra-frame encoding ‘Intra’,whether or not the current block is a luma signal block and the encodingmode of the current block is the LM is determined at step S2110.

If, as a result of the determination, it is determined that the currentblock is not a luma signal and the encoding mode of the current block isnot the LM, an intra-prediction direction mode for the chroma signal isobtained at step S2120. If, as a result of the determination, it isdetermined that the current block is a luma signal and the encoding modeof the current block is the LM, an intra-prediction direction mode for aluma signal is obtained at step S2115.

Here, the intra-prediction direction mode for the luma signal of thecurrent block can be used to derive a frequency transform method for theresidual picture of the luma signal. Here, the intra-predictiondirection mode for the chroma signal of the current block can be used toderive a frequency transform method for the residual picture of thechroma signal.

Furthermore, if the chroma signal of the current block has been encodedin the LM, the intra-prediction direction mode for the luma signal ofthe current block can be used to derive a frequency transform method forthe residual picture of the chroma signal.

Next, whether or not the current block has a 4×4 size (iWidth==4) isdetermined at step S2125. If, as a result of the determination, it isdetermined that the current block does not have a 4×4 size (iWidth==4),integer transform or integer DCT can be applied to frequency transformmethods for the luma signal and the chroma signal of the current blockat step S2160. If, as a result of the determination, it is determinedthat the current block has a 4×4 size (iWidth==4), the intra-predictiondirection mode of the current block is checked.

Here, when deriving a frequency transform method for the luma signal,IntraPredMode(C) is changed into IntraPredMode (i.e., anintra-prediction direction mode for the luma signal). When deriving afrequency transform method for the chroma signal, IntraPredMode(C) ischanged into IntraPredModeC (i.e., an intra-prediction direction modefor the chroma signal).

If the intra-prediction direction mode of the current block is 2 or moreand 10 or less, at step S2130), integer DST can be applied to afrequency transform method for the current block in a horizontaldirection and integer DCT can be applied to a frequency transform methodfor the current block in a vertical direction at step S2145.

In contrast, if the intra-prediction direction mode of the current blockis 0 or is 11 or more and 25 or less at step S2135, integer DST can beapplied to a frequency transform method for the current block in bothhorizontal and vertical directions at step S2150.

In contrast, if the intra-prediction direction mode of the current blockis 26 or more and 34 or less at step S2140, integer DCT can be appliedto a frequency transform method for the current block in a horizontaldirection and integer DST can be applied to a frequency transform methodfor the current block in a vertical direction at step S2155.

If the intra-prediction direction mode of the current block is not 26 ormore and 34 or less at step S2140, integer DCT can be applied to afrequency transform method for the residual pictures of the luma signaland the chroma signal of the current block in both horizontal andvertical directions at step S2160.

A method of incorporating the method according to the example of FIG. 21into a transform process for scaled transform coefficients is asfollows.

A Transform Process for Scaled Transform Coefficients into Which theExample of FIG. 21 has Been Incorporated

In this case, the input is as follows.

-   -   The width of a current transform block: nW    -   The height of the current transform block: nH    -   An array of scaled transform coefficients having an element dij:        (nWxnH) array d

An index for the luma signal and the chroma signal of a current block:cIdx

-   -   If cIdx is 0, it means a luma signal. If cIdx is 1 or cIdx is 2,        it means a chroma signal. Furthermore, if cIdx is 1, it means Cb        in a chroma signal. If cIdx is 2, it means Cr in a chroma        signal.

In this case, the output is as follows.

An array for a residual signal obtained by inversely transforming thescaled transform coefficients: (nWxnH) array r

If an encoding mode ‘PredMode’ for the current block is anintra-prediction mode ‘Intra’ and a value of Log2(nW*nH) is 4,parameters horizTrType and vertTrType according to Table 14 are useddepending on intra-prediction direction modes for a luma signal. In thecase of the chroma signal, the parameters horizTrType and vertTrTypeaccording to Table 14 are used depending on an intra-predictiondirection mode for a chroma signal. If an encoding mode for the chromasignal is Intra_FromLuma (LM), the parameters horizTrType and vertTrTypeaccording to Table 14 are used depending on an intra-predictiondirection mode for the luma signal. If an encoding mode for the chromasignal is not Intra_FromLuma (LM), the parameters horizTrType andvertTrType are set to 0.

TABLE 14 IntraPredMode 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17vertTrType 1 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 horizTrType 1 0 1 1 1 1 11 1 1 1 1 1 1 1 1 1 1 IntraPredMode 18 19 20 21 22 23 24 25 26 27 28 2930 31 32 33 34 vertTrType 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 horizTrType1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0

An inverse transform process is performed on the scaled transformcoefficients using the parameters horizTrType and vertTrType value.First, the sizes nW and nH and scaled transform coefficients array‘nWxnH array d’ of the current block and the parameter horizTrType arereceived, 1-D inverse transform is performed in a horizontal direction,and an array ‘nWxnH array e’ is output as a result of the 1-D inversetransform.

Next, an array ‘nWxnH array e’ is received, and an array ‘nWxnH array g’is derived as in Equation 15 using the array.

g _(ij)=Clip3(−32768,32767,(e _(ij)+64)>>7)   <Equation 15>

Next, the sizes nW and nH and array ‘nWxnH array g’ of the current blockand the parameter vertTrType are received, and 1-D inverse transform isperformed in a vertical direction.

Next, an array ‘nWxnH array r’ for the residual signals is set based oncIdx as in Equation 16.

r _(ij)=(f _(ij)+(1<(shift-1)))>>shift   <Equation 16>

Here, when cIdx is 0, shift=20—BitDepthy. When cIdx is not 0,shift=20—BitDepthc. BitDepth indicates the number of bits (e.g., 8 bits)of a sample for a current picture.

The above-described methods can be differently applied to an applicationscope depending on the size of all blocks, a CU depth, or a TU depth.

A parameter (i.e., information about the size or depth) used todetermine the application scope as described above may be set so that apredetermined value is used in an encoder and a decoder, or apredetermined value may be used depending on a profile or level. Forexample, when an encoder writes a parameter value in a bit stream, adecoder may obtain a value from the bit stream and use the value.

If the application scope is different depending on a CU depth, one ofthe following 3 methods can be used as illustrated in Table 15: A methodA) of applying the application scope to only a depth equal to or greaterthan a specific depth, a method B) of applying the application scope toonly a depth equal to or smaller than a specific depth, and a method C)of applying the application scope to only a specific depth.

Table 15 shows an example of methods of determining the applicationscope in which the methods of the present invention is applied when aspecific CU (or TU) depth is 2. In Table 15, ‘O’ indicates that theapplication scope is applied to a corresponding depth, and ‘X’ indicatesthat the application scope is not applied to a corresponding depth.

TABLE 15 CU (OR TU) DEPTH INDICATING APPLICATION SCOPE METHOD A METHOD BMETHOD C 0 X O X 1 X O X 2 O O O 3 O X X 4 O X X

If the methods of the present invention are not applied to all depths,it may be indicated using a specific indicator (or flag) or may berepresented by signaling a value that is 1 greater than a maximum valueof a CU depth using as a CU depth value indicative of the applicationscope.

Furthermore, the above-described methods can also be applied to a casewhere a luma signal has a different resolution from a chroma signal.

For example, assuming that the size of a chroma signal is ¼ of the sizeof a luma signal (e.g., a luma signal has a 416×240 size and a chromasignal has a 208×120 size), a luma signal 8×8 block is equal to a chromasignal 4×4 block.

FIG. 22 shows an example of a difference in the resolution between aluma block and a chroma block. The example of FIG. 22 illustrates thatthe luma signal is an 8×8 block 2210 and the chroma signal is a 4×4block 2220.

In the example of FIG. 22, the luma signal 2210 can include four 4×4blocks, and each of the 4×4 blocks can have an intra-predictiondirection mode. In contrast, the chroma signal 2220 may not bepartitioned into 2×2 blocks.

In this case, the chroma signal can have one intra-prediction directionmode for the 4×4 block. Here, if the chroma signal has been encoded inIntra_FromLuma (LM) (or if the chroma signal has been encoded in DM(i.e., when an intra-prediction direction mode for the luma signal isused as an intra-prediction direction mode for the chroma signal withoutchange)), one of the four 4×4 blocks can be used as the intra-predictiondirection mode for the luma signal used to derive a frequency transformmethod for the residual picture of the chroma signal.

In order to selectively apply a frequency transform method to theresidual picture of the chroma signal, one of the following 1 to 4 canbe applied to a method of deriving an intra-prediction direction invarious ways.

1. The intra-prediction direction mode in the up-left block of the lumasignal block can be used.

2. The intra-prediction direction mode in the up-right, down-left, ordown-right block of the luma signal block can be used.

3. The mean value or middle value of the four luma signal blocks can beused.

4. The mean value or middle value using intra-prediction direction modesfor the four luma signal blocks of a current block and chroma signalblocks neighboring the current block can be used.

In addition, an intra-prediction direction mode for the chroma signalcan be derived using various methods.

FIG. 23 is another example illustrating a difference in the resolutionbetween a luma block and a chroma block.

As in the example of FIG. 23, a luma signal 2310 can include one 16×16block and can have one intra-prediction mode. In contrast, a chromasignal 2320 can be partitioned into four 4×4 blocks. Here, each of the4×4 blocks can have an intra-prediction direction mode.

Here, if the chroma signal has been encoded in Intra_FromLuma (LM) (orif the chroma signal has been encoded in DM (i.e., when anintra-prediction direction mode for the luma signal is used as anintra-prediction direction mode for the chroma signal without change)),an intra-prediction direction mode for the one luma signal can be usedto derive a frequency transform method for the residual pictures of eachchroma signal. In another embodiment, in order to derive the frequencytransform method for the residual pictures of the chroma signal block,intra-prediction direction modes may be derived from blocks (i.e., lumablocks or chroma blocks) neighboring a current block may be derived andused.

Furthermore, the above-described method can be differently applied to achroma block depending on the size of a luma block. Furthermore, theabove-described method can be differently applied to a luma signalpicture and a chroma signal picture. Furthermore, a frequency transformmethod can be separately applied to a horizontal direction and avertical direction.

Table 16 schematically illustrates a combination of the methods.

TABLE 16 SIZE OF SIZE OF HORI- VER- LUMA CHROMA LUMA CHROMA ZONTAL TICALBLOCK BLOCK APPLIED APPLIED APPLIED APPLIED METHODS 4 (4 × 4, 4 × 2, 2(2 × 2) O or X O or X O or X O or X A 1, 2, . . . 2 × 4) 4 (4 × 4, O orX O or X O or X O or X B 1, 2, . . . 4 × 2, 2 × 4) 8 (8 × 8, 8 × 4, O orX O or X O or X O or X C 1, 2, . . . 4 × 8, 2 × 8, etc.) 16 (16 × 16, 16× 8, O or X O or X O or X O or X D 1, 2, . . . 4 × 16, 2 × 16, etc.) 32(32 × 32) O or X O or X O or X O or X E 1, 2, . . . 8 (8 × 8, 2 (2 × 2)O or X O or X O or X O or X F 1, 2, . . . 8 × 4, 2 × 8, 4 (4 × 4, 4 × 2,O or X O or X O or X O or X G 1, 2, . . . etc.) 2 × 4) 8 (8 × 8, 8 × 4,O or X O or X O or X O or X H 1, 2, . . . 4 × 8, 2 × 8, etc.) 16 (16 ×16, O or X O or X O or X O or X I 1, 2, . . . 16 × 8, 4 × 16, 2 × 16,etc.) 32 (32 × 32) O or X O or X O or X O or X J 1, 2, . . . 16 (16 ×16, 2 (2 × 2) O or X O or X O or X O or X K 1, 2, . . . 8 × 16, 4 (4 ×4, 4 × 2, O or X O or X O or X O or X L 1, 2, . . . 4 × 16, 2 × 4) etc.)8 (8 × 8, 8 × 4, O or X O or X O or X O or X M 1, 2, . . . 4 × 8, 2 × 8,etc.) 16 (16 × 16, O or X O or X O or X O or X N 1, 2, . . . 16 × 8, 4 ×16, 2 × 16, etc.) 32 (32 × 32) O or X O or X O or X O or X O 1, 2, . . .

In the modified methods of Table 16, the method ‘G 1’ is describedbelow. If the size of a luma block is 8 (8×8, 8×4, 2×8, etc.) and thesize of a chroma block is 4 (4×4, 4×2, 2×4), Embodiment 1 (G1—Embodiment 1), Embodiment 2-1, Embodiment 2-2 (G 1—Embodiment 2-1 orEmbodiment 2-2), and Embodiment 3 (G 1—Embodiment 3) of thisspecification can be applied to a luma signal, a chroma signal, ahorizontal signal, and a vertical signal.

In the modified methods of Table 16, the method ‘L 2’ is describedbelow. If the size of a luma block is 16 (16×16, 8×16, 4×16, etc.) andthe size of a chroma block is 4 (4×4, 4×2, 2×4), Embodiment 1 (L2—Embodiment 1), Embodiment 2-1 or Embodiment 2-2 (L 2—Embodiment 2-1 orEmbodiment 2-2), and Embodiment 3 (L 2—Embodiment 3) can be applied to aluma signal, a chroma signal, and a horizontal signal and may not beapplied to a vertical signal.

Here, the above-described methods can be applied to various cases, suchas 1 and 2 below, in addition to a case where a current chroma block hasbeen encoded in ‘Intra_FromLuma’ (LM).

1. In predicting a chroma block, the above-described methods can beapplied to all methods of multiplying a reconstructed luma signal blockby weight and adding an offset to a result of the multiplication.

2. In predicting a chroma block, the above-described methods can beapplied to all methods of predicting a chroma block using a templateshift for reconstructed luma signal and chroma signal blocks. Here, thetemplate shift is used to derive a correlation between a luma signal anda chroma signal.

In addition, the above-described methods can be applied in various ways.

[Embodiment 6] A method and apparatus for integrally determining afrequency transform method and a scanning method for a chroma signal

As described above, a scanning method for the residual signal (orresidual picture) of a chroma signal can be different depending on anintra-prediction direction mode for the chroma signal.

Furthermore, a frequency transform method for the residual signal (orresidual picture) of a chroma signal can be different depending on anintra-prediction direction mode for the chroma signal.

A scanning method for the residual signal (or residual picture) of achroma signal can have different encoding efficiency depending on afrequency transform method for the residual signal of the chroma signal.In contrast, a scanning method and a frequency transform method for theresidual signal (or residual picture) of a chroma signal can be said tohave a specific correlation because the two methods can be differentdepending on an intra-prediction direction mode for the chroma signal.

That is, a scanning method and a frequency transform method for theresidual signal of a chroma signal can have a correlation, and the twomethods can be integrated into an optimal combination. Accordingly, thescanning methods for a chroma signal in Embodiment 1 and Embodiment2-1/Embodiment 2-2 and a frequency transform method for Embodiment 3,Embodiment 4, and Embodiment 5 can be integrally determined as in Table17.

Table 17 shows an example of methods of integrally determining ascanning method and a frequency transform method for the residual signalof a chroma signal according to the present embodiment.

TABLE 17 FREQUENCY TRANSFORM SCANNING METHOD FOR METHOD FOR CHROMASIGNAL CHROMA SIGNAL EM DM LM EM DM LM A O X O X O B X O C O O D O O X EX O F O O O X O X P X O Q O O R O O X S X O T O O

Table 17 shows combinations of methods of integrating determining ascanning method and a frequency transform method for the residual signalof a chroma signal. Here, EM, DM, and LM show methods of encodingintra-prediction direction modes for the chroma signal.

A mode for a chroma signal that is derived by applying anintra-prediction direction mode for a luma signal to an intra-predictiondirection mode for the chroma signal without change is a Derived Mode(DM). A mode for a chroma signal in which an intra-prediction directionmode is actually encoded is an Explicit Mode (EM). The intra-predictiondirection modes of a chroma signal that is encoded in the EM include aplanar mode PLANAR, a mean mode DC, a horizontal mode HOR, a verticalmode VER, and a mode located at an eighth place in a vertical direction(i.e., Ver+8 or No. 34 mode).

Finally, a mode in which a chroma signal is predicted from areconstructed luma signal is Intra_FromLuma (LM).

‘A’, that is, an example of the combinations of Table 17, is describedbelow. In order to determine a frequency transform method for a chromasignal, only the EM and the LM, from among the intra-predictiondirection modes for the chroma signal, are used, but the DM is not used.A frequency transform method is determined using the EM and the LM(e.g., Embodiments 3, 4, 5). Next, in order to determine a scanningmethod for transformed coefficients, a scanning method (e.g.,Embodiments 1 and 2) is determined using the EM and the DM, from amongthe intra-prediction direction modes for the chroma signal.

In this case, a case where a frequency transform method is derived usingthe DM, from among the intra-prediction direction modes for the chromasignal, may be excluded by taking encoding efficiency intoconsideration. For example, in case where encoding is performed in theDM, if an intra-prediction direction mode for a chroma signal isdifferent from an intra-prediction direction mode for a luma signal andthe intra-prediction direction mode for the chroma signal is not presentin the EM (e.g., a planar mode PLANAR, a mean mode DC, a horizontal modeHOR, a vertical mode VER, and a mode located at an eighth place in avertical direction (Ver+8 or No. 34 mode)), the accuracy of theintra-prediction direction mode for the chroma signal encoded in the DMmay be deteriorated because the chroma signal is inevitably encoded inthe DM. For this reason, the DM may not be used to determine a frequencytransform method.

‘D’, that is, another example of the combinations of Table 17, isdescribed below.

In order to determine a frequency transform method for a chroma signal,only the EM and the LM, from among the intra-prediction direction modesfor the chroma signal, are used, but the DM is not used. A frequencytransform method is determined using the EM and the LM (e.g.,Embodiments 3, 4, and 5). Next, in order to determine a scanning methodfor transformed coefficients, a scanning method is determined using allthe EM, DM, and LM, from among the intra-prediction direction modes forthe chroma signal, (e.g., Embodiments 1 and 2).

In this case, a case where a scanning method is derived using the LM,from among the intra-prediction direction modes for the chroma signal,may be included by taking encoding efficiency into consideration. Asdescribed in connection with Embodiments 1 and 2, a chroma signalencoded in the LM can have a correlation with a luma signal.Accordingly, an intra-prediction direction mode for a chroma signalencoded in the LM can be almost the same as an intra-predictiondirection mode of a luma signal. As a result, the intra-predictiondirection mode for the chroma signal encoded in the LM can be used todetermine a scanning method because the intra-prediction direction modefor the chroma signal has a similar characteristic to theintra-prediction direction mode for the luma signal.

‘T’, that is, yet another example of the combinations of Table 17, isdescribed below. In order to determine a frequency transform method fora chroma signal, all the EM, DM, and LM, from among the intra-predictiondirection modes for the chroma signals, are used. A frequency transformmethod is determined using the EM, DM, and LM (e.g., Embodiments 3, 4,and 5). Next, in order to determine a scanning method for transformedcoefficients, a scanning method (e.g., Embodiments 1 and 2) isdetermined using all the EM, DM, and LM, from among the intra-predictiondirection modes for the chroma signal.

As yet further another example of the combinations of Table 17, if the‘T’ method is modified, the EM, DM, and LM, from among theintra-prediction direction mode for a chroma signal, can be used todetermine a frequency transform method for the chroma signal. Here, afrequency transform method can be determined (e.g., Embodiments 3, 4,and 5) only when an intra-prediction direction mode for a real andreconstructed chroma signal, from among the EM, DM, and LM, is the EM(e.g., a planar mode PLANAR, a mean mode DC, a horizontal mode HOR, avertical mode VER, or a mode located at an eighth place in a verticaldirection (Ver+8 or No. 34 mode)). Furthermore, all the EM, DM, and LMcan be used to determine a scanning method for transformed coefficients.Here, a scanning method (e.g., Embodiments 1 and 2) can be used onlywhen an intra-prediction direction mode for a real and reconstructedchroma signal, from among the EM, DM, and LM, is the EM (e.g., a planarmode PLANAR, a mean mode DC, a horizontal mode HOR, a vertical mode VER,or a mode located at an eighth place in a vertical direction (Ver+8 orNo. 34 mode)).

[Embodiment 7] A method and apparatus for integrating tables fordetermining a frequency transform method and a scanning method for achroma signal

It can be said that a scanning method and a frequency transform methodfor the residual signal (or residual picture) of a chroma signal have aclose correlation because the two methods can be different depending onan intra-prediction direction mode for the chroma signal.

Therefore, in the present embodiment, tables used to determine the twomethods can be integrated and used depending on an intra-predictiondirection mode.

Table 18 shows an example of frequency transform methods and scanningmethods derived based on intra-prediction direction modes according tothe present embodiment.

TABLE 18 A 2~5 6~9 10 11~14 15~17 18 19~21 22~25 26 27~30 31~34 VerticalFrequency DCT DST DST Transform Method Horizontal Frequency DST DCTTransform Method SCANNING DIAG VER DIAG HOR DIAG METHOD

Furthermore, Table 19 shows another example of frequency transformmethods and scanning methods that are derived based on intra-predictiondirection modes according to the present embodiment.

TABLE 19 B 2~5 6~9 10 11~14 15~17 18 19~21 22~25 26 27~30 31~34 VerticalFrequency DST DCT DST DST DST Transform Method Horizontal Frequency DSTDCT Transform Method SCANNING DIAG VER DIAG HOR DIAG METHOD

Table 20 shows yet another example of frequency transform methods andscanning method that are derived based on intra-prediction directionmodes according to the present embodiment.

TABLE 20 C 2~5 6~9 10 11~14 15~17 18 19~21 22~25 26 27~30 31~34 VerticalFrequency DCT DCT DCT DST DCT Transform Method Horizontal Frequency DSTDCT Transform Method SCANNING DIAG VER DIAG HOR DIAG METHOD

Table 21 shows yet further another example of frequency transformmethods and scanning methods that are derived based on intra-predictiondirection modes according to the present embodiment.

TABLE 21 H − − H − V − − V − − (4) (4) H H + (4) H + + (3) D (3) (4) VV + (4) V + + (4) D 2~5 6~9 10 11~14 15~17 18 19~21 22~25 26 27~30 31~34Vertical DCT DCT DST Frequency Transform Method Horizontal DST DCTFrequency Transform Method SCANNING DIAG VER DIAG HOR DIAG METHOD

Tables 18 to 21 shows examples in which the tables of the frequencytransform methods and the scanning methods derived based on theintra-prediction direction modes are integrated.

For example, referring to Table 19, if an intra-prediction directionmode is 2-5, DST is used as vertical and horizontal frequency transformmethods and up-right diagonal scanning DIAG is used as a scanningmethod. If an intra-prediction direction mode is 6∧14, DCT is used as avertical frequency transform method, DST is used as a horizontalfrequency transform method, and VER (i.e., vertical) is used as ascanning method.

If an intra-prediction direction mode is 15˜21, DST is used as verticaland horizontal frequency transform methods and DIAG (i.e., up-rightdiagonal scanning) is used as a scanning method. If an intra-predictiondirection mode is 22˜30, DST is used as a vertical frequency transformmethod, DCT is used as a horizontal frequency transform method, and HOR(i.e., horizontal) is used as a scanning method.

If an intra-prediction direction mode is 31˜34, DST is used as verticaland horizontal frequency transform methods and DIAG (i.e., up-rightdiagonal) is used as a scanning method.

If a frequency transform method and a scanning method are taken intoconsideration based on an intra-prediction direction mode as in Table19, there is an advantage in that unity can be implemented depending onVER and HOR unlike in Table 18.

Here, the above-described methods can also be applied to a case 1 or 2below in addition to a case where a current chroma block has beenencoded in the LM (Intra_FromLuma).

1. In predicting a chroma block, the above-described methods can beapplied to all methods of multiplying a reconstructed luma signal blockby weight and adding an offset to a result of the multiplication.

2. In predicting a chroma block, the above-described methods can beapplied to all methods of predicting a chroma block using a templateshift for reconstructed luma signal and chroma signal blocks. Here, thetemplate shift is used to derive a correlation between a luma signal anda chroma signal.

In addition, the above-described methods can be applied in various ways.

FIG. 24 shows another example of an encoding apparatus according to thepresent invention.

Referring to FIG. 24, the encoding apparatus 2400 includes a predictionmode specification module 2410 and a scanning module 2420.

The prediction mode specification module 2410 specifies a predictionmode applied to a current block. For example, the prediction modespecification module 2410 can specify an intra-prediction mode that hasbeen applied to the luma sample and/or the chroma sample of a currentblock.

Here, the prediction mode specification module 2410 may be placed infront of the intra-prediction module of FIG. 1 and configured to specifyan intra-prediction mode to be applied to a current block or may beplaced in the rear of the intra-prediction module and configured tooutput information indicative of an intra-prediction mode applied to acurrent block.

The intra-prediction mode is the same as that described with referenceto FIG. 6.

The scanning module 2420 can scan the chroma signal and/or the lumasignal of a current block. Here, the scanning module 2420 can determinea scanning type to be applied to the current block in response to anintra-prediction mode for the current block.

A detailed method of determining a scanning type (or method) to beapplied to a current block has been described in detail in connectionwith the embodiments of this specification.

FIG. 25 shows another example of a decoding apparatus according to thepresent invention.

Referring to FIG. 25, the decoding apparatus 2500 includes a predictionmode specification module 2510 and a scanning module 2520.

The prediction mode specification module 2510 specifies a predictionmode applied to a current block. For example, the prediction modespecification module 2510 can specify an intra-prediction mode appliedto the luma sample and/or the chroma sample of a current block.

The intra-prediction mode is the same as that described with referenceto FIG. 6.

The scanning module 2520 can scan the chroma signal and/or the lumasignal of a current block. Here, the scanning module 2520 can determinea scanning type to be applied to the current block in response to anintra-prediction mode for the current block.

A detailed method of determining a scanning type (or method) to beapplied to a current block has been described in detail in connectionwith the embodiments of this specification.

In the above exemplary system, although the methods have been describedbased on the flowcharts in the form of a series of steps or blocks, thepresent invention is not limited to the sequence of the steps, and someof the steps can be performed in a different order from that of othersteps or can be performed simultaneous to other steps. Furthermore, theaforementioned embodiments include various examples. For example, acombination of some embodiments should also be understood as anembodiment of the present invention.

1. A method for decoding a video signal, comprising: deriving an intraprediction mode of a current block; determining a transform type of thecurrent block based on the intra prediction mode; determining a scantype of the current block, wherein the scan type of the current block isdetermined to be a first scan type, a second scan type or a third scantype, the second scan type being more vertical than the first scan typeand the third scan type being more horizontal than the first scan type;obtaining transform coefficients of the current block based on the scantype of the current block; and performing an inverse-transform on thetransform coefficients based on the transform type of the current block,wherein the scan type of the current block is determined to be the firstscan type when the transform type of the current block is a firsttransform type, the scan type of the current block is determined to bethe second scan type when the transform type of the current block is asecond transform type, and the scan type of the current block isdetermined to be the third scan type when the transform type of thecurrent block is a third transform type.
 2. The method of claim 1,wherein: the first transform type represents that a combination of avertical type and a horizontal type for the transform type of thecurrent block is a first transform type combination, the secondtransform type represents that the combination of the vertical type andthe horizontal type for the transform type of the current block is asecond transform type combination, and the third transform typerepresents that the combination of the vertical type and the horizontaltype for the transform type of the current block is a third transformtype combination.
 3. The method of claim 2, wherein: the vertical typefor the transform type of the current block is determined to be DiscreteCosine Transform (DCT) or Discrete Sine Transform (DST), and thehorizontal type for the transform type of the current block isdetermined to be DCT or DST.
 4. The method of claim 3, wherein: thefirst transform type combination represents that the vertical type isthe DST and the horizontal type is the DST, the second transform typecombination represents that the vertical type is the DCT and thehorizontal type is the DST, and the third transform type combinationrepresents that the vertical type is the DST and the horizontal type isthe DCT.
 5. The method of claim 1, wherein determining the transformtype of the current block comprises: determining a transform type of thecurrent block further based on the intra prediction mode, and a colorcomponent the current block.
 6. The method of claim 5, wherein thetransform type of the current block is a type which is determined whenthe color component the current block is a luma.
 7. The method of claim1, wherein determining the transform type of the current blockcomprises: determining the transform type of the current block furtherbased on the intra prediction mode and a transform size of the currentblock.
 8. The method of claim 7, wherein the transform type of thecurrent block is a type which is determined when the transform size ofthe current block is 4×4.
 9. The method of claim 1, further comprising:obtaining prediction mode information of the current block, theprediction mode information specifying whether the current block is aninter-coded block or an intra-coded block.
 10. The method of claim 9,wherein deriving the intra prediction mode of the current blockcomprises: deriving the intra prediction mode of the current block inresponse to the current block being the intra-coded block.
 11. A methodfor decoding a video signal, comprising: determining a transform type ofa current block based on a color component of the current block;determining a scan type of the current block, wherein the scan type ofthe current block is determined to be a first scan type, a second scantype or a third scan type, the second scan type being more vertical thanthe first scan type and the third scan type being more horizontal thanthe first scan type; obtaining transform coefficients of the currentblock based on the scan type; and performing an inverse-transform on thetransform coefficients based on the transform type of the current block,wherein if the color component of the current block is a chromacomponent, the transform type of the current block is determined usingan intra prediction mode for the chroma component of the current blockand a lookup table specifying transform types corresponding to intraprediction modes.
 12. The method of claim 11, wherein if the colorcomponent of the current block is a luma component, the transform typeof the current block is determined not using the lookup table.
 13. Themethod of claim 11, wherein: a vertical type for the transform type ofthe current block is determined to be Discrete Cosine Transform (DCT) orDiscrete Sine Transform (DST), and a horizontal type for the transformtype of the current block is determined to be DCT or DST.
 14. The methodof claim 13, wherein: the scan type is determined to be the first scantype when the transform type of the current block is a first transformtype, the scan type is determined to be the second scan type when thetransform type of the current block is a second transform type, and thescan type is determined to be the third scan type when the transformtype of the current block is a third transform type.
 15. The method ofclaim 14, wherein: the first transform type represents that acombination of a vertical type and a horizontal type for the transformtype of the current block is a first transform type combination, thesecond transform type represents that the combination of the verticaltype and the horizontal type for the transform type of the current blockis a second transform type combination, and the third transform typerepresents that the combination of the vertical type and the horizontaltype for the transform type of the current block is a third transformtype combination.
 16. The method of claim 11, wherein determining thetransform type of the current block comprises: determining the transformtype of the current block further based on the intra prediction mode anda transform size of the current block.
 17. The method of claim 11,further comprising: obtaining prediction mode information of the currentblock, the prediction mode information specifying whether the currentblock is an inter-coded block or an intra-coded block.
 18. The method ofclaim 17, wherein the intra prediction mode for the chroma component ofthe current block is derived in response to the current block being theintra-coded block.
 19. A method for encoding a video signal, comprising:determining a transform type of a current block based on a colorcomponent of the current block; determining a scan type of the currentblock, wherein the scan type of the current block is determined to be afirst scan type, a second scan type or a third scan type, the secondscan type being more vertical than the first scan type and the thirdscan type being more horizontal than the first scan type; transforming aresidual block based on the transform type of the current block togenerate transform coefficients; and scanning the transform coefficientsof the current block based on the scan type, wherein if the colorcomponent of the current block is a chroma component, the transform typeof the current block is determined using an intra prediction mode forthe chroma component of the current block and a lookup table specifyingtransform types corresponding to intra prediction modes.
 20. The methodof claim 19, wherein if the color component of the current block is aluma component, the transform type of the current block is determinednot using the lookup table.